satellite - An object in orbit around a larger object, especially when held in orbit by gravitational force.

semipermeable - A property of some membranes which selectively allows some molecules to pass through it, but bars the passage of others.

set - (1) A collection of objects or concepts that share a given property, or are related in some way; (2) A collection of objects or concepts grouped together; (3) A mathematical entity that obeys the axioms (assumptions) of formal set theory. The null or empty set is considered to be a set; other sets are constructed using the null set plus operations described in the axioms. Sets containing exactly the same elements are considered to be the same set. See also subset, proper subset, finite, infinite.

set theory - The mathematical treatment of sets, operations on sets, and relationships among sets. Finite set theory is essentially an extension of basic logic. Infinite set theory adds an induction operation for producing infinite sets, and requires use of a number of axioms (assumptions) to avoid contradictions that can arise without such precautions, especially if a set were to be allowed to contain itself, even if only indirectly. The axioms are designed specifically to prevent such self-referencing from occurring. See cardinal number.

sexual - Descriptive of organisms that initiate reproduction by the fusion of gametes.

shell - see electron shell.

sigma particle - Any of three unstable subatomic particles in the baryon family, having a mass of 2,328 to 2,343 times that of the electron and a positive, neutral, or negative electric charge. Sigma particles are each made of three quarks, of which one is a strange quark and the other two are up or down quarks.

Silurian Period - (440 to 410 million years ago) Primitive vegetation, invertebrates, jawless fish, and the first insects characterize this period. See geologic time.

skew - Neither parallel nor intersecting. Two lines are skew if there's no one plane that contains both.

Sol - The yellow dwarf star which planet Earth orbits.

solar cell - Any device that directly converts the energy in light into electrical energy.

solid - Matter that resists a change in shape; matter in which the atoms and molecules retain the same positions relative to each other. This is one of the four states in which matter made of atoms can exist; the others are liquid, gas, and plasma. The matter inside a neutron star, called neutronium, is too dense for atoms to exist; lacking atoms, it is not included among the normal states of matter. See also melting point.

solute - Any substance that is dissolved in another substance, which is called a solvent.

solution - A mixture in which one or more substances are dissolved in another substance.

solvent - Any substance in which another substance can be dissolved.

space - (1) A set of points with fixed distances between any two of them, such that the rules of geometry apply (there can be different kinds of space for different kinds of geometry); (2) A space with no endpoints or edge - from any point in the space, all directions are still in that space; (3) A set of points indistinguishable from the earlier definitions at lengths or distances much greater than some very small value, presumed to be around the Planck length; (4) The three dimensional space in which we live, presumed to not have end or edge, possibly fitting one of the earlier definitions; (5) The space above the atmosphere of planet Earth, sometimes called outer space, or sometimes space excluding that inside or near a planet or atmosphere; (6) A space of a given number of dimensions, sometimes called n-space where n is a number (1, 2, 3, ... ); (7) The volume, or measure of extent, of a space or subset of a space (for 1 dimension this is called length, for 2 dimensions area, for 3 dimensions, volume).

space-time - (1) A space combined with a time dimension; (2) The space-time in which we live, which has three visible space dimensions and one visible time dimension.

special relativity (abbr. SR) - Einstein's theory of space, time, and motion.

species - Biology. Organisms within a genus that are related so closely that fertile offspring are produced by breeding are considered to be of the same species. In some cases, where two species are just beginning to emerge, the offspring from cross-mating's may not be fertile. When hybrid offspring are entirely sterile, or nearly so, the parents are considered to be of different species.  This criterion is not absolute, so whether or not the parents are of different species is sometimes a matter for debate. Among plants, judging sameness of species is somewhat different because two types of plants may mate successfully, but produce offspring that have the characteristics of only one parental type. The sameness of extinct species -- known only from fossils -- and of asexual species, must be decided on degrees of similarity.  See taxon.

spin - The amount of angular momentum associated with a subatomic particle or nucleus. Fundamental particles are limited to specific values of spin. For instance, fermion spin is always a whole number plus one half, and boson spin is always a whole number.

spiral galaxy - A galaxy that appears at a distance to have spiral shaped "arms" extending from a central mass of stars. The arms may simply be regions of brighter stars rather than of greater density of stars.

star - An object so large that its interior becomes hot enough to support fusion reactions.

stratosphere - The layer of Earth's atmosphere between the troposphere and the ionosphere. It starts at around 11 to 16 km altitude, and goes up to around 40 km. Within the stratosphere temperature does not vary predictably with altitude, and clouds of condensed water vapor are rare.

strong force - One of the four known fundamental forces, also known as the quark color force, and sometimes called the nuclear force because of its effect in holding atomic nuclei together.

subatomic particle - A particle that is contained in an atom, or that is smaller than an atom. These include baryons such as protons and neutrons, leptons such as electrons, the neutrinos associated with leptons, and quarks which are the components of baryons.

sublimate - To change directly from solid to gas, without passing through a liquid stage. Comparable to the evaporation of a liquid.

subset - A set all of whose elements belong to another set. See also proper subset.

substance - (1) Anything that has mass and occupies space;  (2) A material of a particular kind or composition.  Everyday objects are all made of some combination of substances. In chemistry, substances are composed of molecules all having the same number of atoms of each element present, in the same arrangement.

superimpose (verb; especially of waves) - The act of causing a superposition.

superposition (especially of waves) - The occupying of the same space by waves or quantum particles without losing their separate identities.

superstring theory - A proposed new theory, which describes fundamental particles as vibrating strings rather than dimensionless points. That is, they have a nonzero length. If this theory works it will include both quantum theory and general relativity. Both of these work under "normal" conditions; the new theory should work in situations in which the old theories conflict with each other, such as around black holes or the big bang.

surface - A boundary between two regions, especially the boundary between a solid or liquid material and the neighboring material or region.

synapse - The junction across which a nerve impulse passes from an axon terminal to a neuron, a muscle cell, or a gland cell.