Fahrenheit - A temperature scale in which 32° is defined as the freezing point of water, and 212° is defined as the boiling point of water, with both of these measured at one atmosphere of pressure. See Celsius.
family (plural families) - Biology. A group of related organisms ranking below order and above genus in degree of relationship, e.g., the Primate order includes the Pongidae (ape) family which includes several Genera including chimpanzees, gorillas, gibbons, and orangutans. See taxon.
femto- (abbr. f) - Prefix meaning one quadrillionth (10-15) of something, especially a unit of measure. See metric system.
fermion - A particle with a fractional spin (1/2, 1 1/2, 2 1/2, ... ). Fermions obey the Pauli exclusion principle - two of them cannot occupy the same location and quantum state at the same time. Leptons (electrons, neutrinos, etc.) and baryons (protons, neutrons, etc.) are fermions, while energy particles such as photons are bosons.
field - Something that affects the properties of space over an extended (possibly infinite) volume. Fields are generally described mathematically by assigning a value to each point in the affected space. The value may be a simple number, a complex number, a vector, or other mathematical entity. Examples are the electromagnetic force field, and the gravitational force field.
finite - (1) Having a quantity or size that can be reached or exceeded by counting, or by measurement; a quantity that can be expressed using only integers or real numbers; not infinite. (2) Set theory Describing a set that is larger in size than all proper subsets of itself. For example, the set of whole numbers from 1 to 10 is finite, since any proper subset has fewer than ten elements.
fission - Splitting of an object into two or more objects, especially in the case of the nucleus of an atom. Also known as nuclear fission or atomic fission.
flat space - Space in which gravity is weak enough to be disregarded for a given discussion or experiment.
fluid - Matter that does not retain a definite shape unless constrained. Liquid, gas, and plasma are all fluids.
fluidity - The property of being able to change shape when under pressure. See viscosity.
force - Acceleration or potential acceleration of a mass, by changing its speed or direction of motion. A force acts in a specific direction, so when more than one force acts on a particle one force can cancel or partially cancel another. Thus there can be force without overall acceleration. Net force can be computed by the formula f = ma (f = force vector, m = mass, a = acceleration vector).
force field - A field in which a force is associated with each point in the field. For example, the magnitude and direction of the force at each point in an electromagnetic field describes how a charged particle will react to that field.
force particle - A particle that is produced or absorbed by another particle when it's motion is affected by a force; a particle exchanged between two other particles when there is a force between them.
founder effect - Biology. The result of producing a new population from a few individuals of a previous population (a genetic bottleneck), in which those individuals differ genetically from the average of the original population, and those differences become normal for the new population. Genes rare in the original may be common in the new, or genes previously common may be absent. In statistics this is called "sampling error".
frame of reference - A coordinate system, or map, based on taking one point to be the origin (zero point), and taking lines perpendicular to each other as reference lines for use in identifying points in space. Each of the reference lines is called an "axis". If the origin and reference lines are either stationary or moving at constant velocity the frame is called an "inertial frame of reference".
freeze - To change from liquid or gas into a solid.
freezing point - Temperature at which a liquid changes to a solid. Usually equal to the melting point for a given substance.
frequency - (1) The number of repetitions of a wave per unit time. In particle physics this is generally expressed as number per second. Frequency is inversely related to wavelength. (2) The number of repetitions of a wave per unit distance.
FTL - Abbreviation for "faster than light", referring to anything moving faster than the speed of light in a vacuum in flat space (i.e., when its not slowed down by matter or gravity or other forces).
fuel cell - Any device that converts the chemical energy of a fuel directly into electricity.
fundamental particle - A particle that isn't a combination of lesser particles. Current theory is that electrons and quarks are the fundamental particles out of which normal matter is made. Neutrinos are fundamental particles associated with electrons and the other leptons. See elementary particle.
fusion - Combining of two or more objects into one, especially in the case of atomic nuclei combining into a single heavier nucleus. Also known as atomic fusion or nuclear fusion.