**Definition**— a precise and unambiguous description of the meaning of a mathematical term. It characterizes the meaning of a word by giving all the properties and only those properties that must be true.

**Theorem**— a mathematical statement that is proved using rigorous mathematical reasoning. In a mathematical paper, the term theorem is often reserved for the most important results.

**Lemma**— a minor result whose sole purpose is to help in proving a theorem. It is a stepping stone on the path to proving a theorem. Very occasionally lemmas can take on a life of their own (Zorn’s lemma, Urysohn’s lemma, Burnside’s lemma, Sperner’s lemma).

**Corollary**— a result in which the (usually short) proof relies heavily on a given theorem (we often say that “this is a corollary of Theorem A”).

**Proposition**— a proved and often interesting result, but generally less important than a theorem.

**Conjecture**— a statement that is unproved, but is believed to be true (Collatz conjecture, Goldbach conjecture, twin prime conjecture).

**Claim**— an assertion that is then proved. It is often used like an informal lemma.

**Axiom**/

**Postulate**— a statement that is assumed to be true without proof. These are the basic building blocks from which all theorems are proved (Euclid’s five postulates, Zermelo-Fraenkel axioms, Peano axioms).

**Identity**— a mathematical expression giving the equality of two (often variable) quantities (trigonometric identities, Euler’s identity).

**Paradox**— a statement that can be shown, using a given set of axioms and definitions, to be both true and false. Paradoxes are often used to show the inconsistencies in a flawed theory (Russell’s paradox). The term paradox is often used informally to describe a surprising or counterintuitive result that follows from a given set of rules (Banach-Tarski paradox, Alabama paradox, Gabriel’s horn).