Here are some definitions (from a google search, with additions from this thread). Please note the definitions of "theory":
Theory: a well-substantiated explanation of some aspect of the natural world; an organized system of accepted knowledge that applies in a variety of circumstances to explain a specific set of phenomena; "theories can incorporate facts and laws and tested hypotheses." Addendum: "Theories do not grow up to be laws. Theories explain laws." (Courtesy of VadeRetro.)
Theory: A scientifically testable general principle or body of principles offered to explain observed phenomena. In scientific usage, a theory is distinct from a hypothesis (or conjecture) that is proposed to explain previously observed phenomena. For a hypothesis to rise to the level of theory, it must predict the existence of new phenomena that are subsequently observed. A theory can be overturned if new phenomena are observed that directly contradict the theory. [Source]
When a scientific theory has a long history of being supported by verifiable evidence, it is appropriate to speak about "acceptance" of (not "belief" in) the theory; or we can say that we have "confidence" (not "faith") in the theory. It is the dependence on verifiable data and the capability of testing that distinguish scientific theories from matters of faith.
Hypothesis: a tentative theory about the natural world; a concept that is not yet verified but that if true would explain certain facts or phenomena; "a scientific hypothesis that survives experimental testing becomes a scientific theory"; "he proposed a fresh theory of alkalis that later was accepted in chemical practices."
Proof: Except for math and geometry, there is little that is actually proved. Even well-established scientific theories can't be conclusively proved, because--at least in principle--a counter-example might be discovered. Scientific theories are always accepted provisionally, and are regarded as reliable only because they are supported (not proved) by the verifiable facts they purport to explain and by the predictions which they successfully make. All scientific theories are subject to revision (or even rejection) if new data are discovered which necessitates this.
Law: a generalization that describes recurring facts or events in nature; "the laws of thermodynamics."
Model: a simplified representation designed to illuminate complex processes; a hypothetical description of a complex entity or process; a physical or mathematical representation of a process that can be used to predict some aspect of the process; a representation such that knowledge concerning the model offers insight about the entity modelled.
Speculation: a hypothesis that has been formed by speculating or conjecturing (usually with little hard evidence). When a scientist speculates he is drawing on experience, patterns and somewhat unrelated things that are known or appear to be likely. This becomes a very informed guess.
Conjecture: speculation: a hypothesis that has been formed by speculating or conjecturing (usually with little hard evidence); guess: a message expressing an opinion based on incomplete evidence; reasoning that involves the formation of conclusions from incomplete evidence.
Guess: an opinion or estimate based on incomplete evidence, or on little or no information.
Assumption: premise: a statement that is assumed to be true and from which a conclusion can be drawn; "on the assumption that he has been injured we can infer that he will not to play"
Impression: a vague or subjective idea in which some confidence is placed; "his impression of her was favorable"; "what are your feelings about the crisis?"; "it strengthened my belief in his sincerity"; "I had a feeling that she was lying."
Opinion: a personal belief or judgment that is not founded on proof or certainty.
Observation: any information collected with the senses.
Data: Individual measurements; facts, figures, pieces of information, statistics, either historical or derived by calculation, experimentation, surveys, etc.; evidence from which conclusions can be inferred.
Fact: when an observation is confirmed repeatedly and by many independent and competent observers, it can become a fact.
Truth: This is a word best avoided entirely in physics [and science] except when placed in quotes, or with careful qualification. Its colloquial use has so many shades of meaning from it seems to be correct to the absolute truths claimed by religion, that its use causes nothing but misunderstanding. Someone once said "Science seeks proximate (approximate) truths." Others speak of provisional or tentative truths. Certainly science claims no final or absolute truths. Source.
Science: a method of learning about the world by applying the principles of the scientific method, which includes making empirical observations, proposing hypotheses to explain those observations, and testing those hypotheses in valid and reliable ways; also refers to the organized body of knowledge that results from scientific study.
Religion: Theistic: 1. the belief in a superhuman controlling power, esp. in a personal God or gods entitled to obedience and worship. 2. the expression of this in worship. 3. a particular system of faith and worship.
Religion: Non-Theistic: The word religion has many definitions, all of which can embrace sacred lore and wisdom and knowledge of God or gods, souls and spirits. Religion deals with the spirit in relation to itself, the universe and other life. Essentially, religion is belief in spiritual beings. As it relates to the world, religion is a system of beliefs and practices by means of which a group of people struggles with the ultimate problems of human life.
Belief: any cognitive content (perception) held as true; religious faith.
Faith: the belief in something for which there is no material evidence or empirical proof; acceptance of ideals, beliefs, etc., which are not necessarily demonstrable through experimentation or observation. A strong belief in a supernatural power or powers that control human destiny.
Dogma: a religious doctrine that is proclaimed as true without evidence.
Some good definitions, as used in physics, can be found: Here.
Based on these, evolution is a theory.
[Last revised 9/26/06]