| Absolute humidity (or Saturation value) : | The maximum amount of water vapor, which could be present in 1 m³ of the air at any given temperature, is called absolute humidity. |
| Absolute magnitude : | A classification scheme, which compensates for the distance, differences to stars. It calculates the brightness that stars would appear to have if they were all at a defined, standard distance of 10 parsec |
| Absolute scale : | Temperature scale set so that zero is at the theoretical lowest temperature possible. This would occur when all random motion of molecules has ceased |
| Absolute zero : | The theoretical lowest temperature possible, which occurs when all random motion of molecules has ceased |
| Acceleration due to gravity : | The acceleration produced in a body due to the earth's attraction is called acceleration due to gravity. It is denoted by the letter g. Its SI unit is m/s². On the surface of the earth, its average value is 9.8m/s². The value of g on the surface of the e |
| Acceleration : | The rate of change of velocity of a moving object is called its acceleration. The SI units of acceleration are m / s². By definition, this change in velocity can result from a change in speed, a change in direction, or a combination of changes in speed a |
| Adiabatic cooling : | The decrease in temperature of an expanding gas that involves no additional heat flowing out of the gas. It is the cooling from the energy lost by expansion |
| Adiabatic heating : | The increase in temperature of compressed gas that involves no additional heat flowing into the gas. It is heating from the energy gained by compression |
| Air mass : | A large, more or less uniform body of air with nearly the same temperature and moisture conditions throughout |
| Allotropic forms : | Elements that can have several different structures with different physical properties-for example, graphite and diamond are two allotropic forms of carbon |
| Alpha particle : | The nucleus of a helium atom (two protons and two neutrons) emitted as radiation from a decaying heavy nucleus; also known as an alpha ray |
| Alternating current : | An electric current that first moves one direction, then the opposite direction with a regular frequency |
| Amp : | Unit of electric current. It is equivalent to coulomb/sec. |
| Ampere : | Full name of the unit Amp |
| Amplitude (of waves) : | The maximum displacement of particles of the medium from their mean positions during the propagation of a wave is called the amplitude of the wave. |
| Amplitude (of an oscillation) : | The maximum displacement of a body from its mean position during an oscillatory motion is called the amplitude of oscillation. |
| Angle of incidence : | Angle of an incident (arriving) ray or particle to a surface; measured from a line perpendicular to the surface (the normal) |
| Angle of reflection : | Angle of a reflected ray or particle from a surface; measured from a line perpendicular to the surface (the normal) |
| Angular Acceleration : | The rate of change of angular velocity of a body moving along a circular path is called its angular acceleration. Angular acceleration is denoted by a. |
| Angular Displacement : | The angle described at the center of the circle by a moving body along a circular path is called angular displacement. It is measured in radians. |
| Angular Momentum Quantum Number : | From quantum mechanics model of the atom, one of four descriptions of the energy state of an electron wave. The quantum number describes the energy sublevels of electrons within the main energy levels of an atom |
| Angular Velocity : | The rate of change of angular displacement is called angular velocity. |
| Astronomical unit : | The radius of the earth's orbit is defined as one astronomical unit (A.U.) |
| Atom : | The smallest unit of an element that can exist alone or in combination with other elements |
| Atomic mass unit : | Relative mass unit (u) of an isotope based on the standard of the carbon-12 isotope, which is defined as a mass of exactly 12.00 u; one atomic mass unit (I u) is 1/12 the mass of a carbon-12 atom |
| Atomic number : | The number of protons in the nucleus of an atom |
| Atomic Weight : | Weighted average of the masses of stable isotopes of an element as they occur in nature, based on the abundance of each isotope of the element and the atomic mass of the isotope compared to carbon-12 |
| Avogadro's Number : | The number of carbon-12 atoms in exactly 12.00 g of C that is 6.02 x 10^23 atoms or other chemical units. It is the number of chemical units in one mole of a substance |
| Axis : | The imaginary line about which a planet or other object rotates |
| Absolute humidity (or Saturation value) : | The maximum amount of water vapor, which could be present in 1 m³ of the air at any given temperature, is called absolute humidity. |
| Absolute magnitude : | A classification scheme, which compensates for the distance, differences to stars. It calculates the brightness that stars would appear to have if they were all at a defined, standard distance of 10 parsec |
| Absolute scale : | Temperature scale set so that zero is at the theoretical lowest temperature possible. This would occur when all random motion of molecules has ceased |
| Absolute zero : | The theoretical lowest temperature possible, which occurs when all random motion of molecules has ceased |
| Acceleration due to gravity : | The acceleration produced in a body due to the earth's attraction is called acceleration due to gravity. It is denoted by the letter g. Its SI unit is m/s². On the surface of the earth, its average value is 9.8m/s². The value of g on the surface of the earth increases in going towards the poles from the equator. The acceleration due to gravity of the earth decreases with altitude and with depth inside the earth. The value of g at the center of the earth is zero. |
| Acceleration : | The rate of change of velocity of a moving object is called its acceleration. The SI units of acceleration are m / s². By definition, this change in velocity can result from a change in speed, a change in direction, or a combination of changes in speed and direction |
| Adiabatic cooling : | The decrease in temperature of an expanding gas that involves no additional heat flowing out of the gas. It is the cooling from the energy lost by expansion |
| Adiabatic heating : | The increase in temperature of compressed gas that involves no additional heat flowing into the gas. It is heating from the energy gained by compression |
| Air mass : | A large, more or less uniform body of air with nearly the same temperature and moisture conditions throughout |
| Allotropic forms : | Elements that can have several different structures with different physical properties-for example, graphite and diamond are two allotropic forms of carbon |
| Alpha particle : | The nucleus of a helium atom (two protons and two neutrons) emitted as radiation from a decaying heavy nucleus; also known as an alpha ray |
| Alternating current : | An electric current that first moves one direction, then the opposite direction with a regular frequency |
| Amp : | Unit of electric current. It is equivalent to coulomb/sec. |
| Ampere : | Full name of the unit Amp |
| Amplitude (of waves) : | The maximum displacement of particles of the medium from their mean positions during the propagation of a wave is called the amplitude of the wave. |
| Amplitude (of an oscillation) : | The maximum displacement of a body from its mean position during an oscillatory motion is called the amplitude of oscillation. |
| Angle of incidence : | Angle of an incident (arriving) ray or particle to a surface; measured from a line perpendicular to the surface (the normal) |
| Angle of reflection : | Angle of a reflected ray or particle from a surface; measured from a line perpendicular to the surface (the normal) |
| Angular Momentum Quantum Number : | From quantum mechanics model of the atom, one of four descriptions of the energy state of an electron wave. The quantum number describes the energy sublevels of electrons within the main energy levels of an atom |
| Angular Velocity : | The rate of change of angular displacement is called angular velocity. |
| Astronomical unit : | The radius of the earth's orbit is defined as one astronomical unit (A.U.) |
| Atom : | The smallest unit of an element that can exist alone or in combination with other elements |
| Atomic mass unit : | Relative mass unit (u) of an isotope based on the standard of the carbon-12 isotope, which is defined as a mass of exactly 12.00 u; one atomic mass unit (I u) is 1/12 the mass of a carbon-12 atom |
| Atomic Weight : | Weighted average of the masses of stable isotopes of an element as they occur in nature, based on the abundance of each isotope of the element and the atomic mass of the isotope compared to carbon-12 |
| Avogadro's Number : | The number of carbon-12 atoms in exactly 12.00 g of C |
| Axis : | The imaginary line about which a planet or other object rotates |