A tag is a keyword or label that categorizes your question with other, similar questions. Using the right tags makes it easier for others to find and answer your question.

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a major branch of algebraic number theory that studies abelian extensions of global and local fields.
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for questions about (not necessarily periodic) tilings of metric spaces, their combinatorial, topological and dynamical properties, as well as basic definitions and concepts.
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Questions relating to the golden ratio $\varphi = \frac{1+\sqrt{5}}{2}$
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for questions regarding the Newton–Raphson method. In numerical analysis the Newton–Raphson method is a method for finding successively better approximations to the roots (or zeroes) of a …
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a mathematical object which formalizes some of the essential features of the Cartesian product $X\times G$ of a space $X$ with a group $G$.
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For topics of a combinatorial character in set theory. Topics belonging to "combinatorial set theory" may be tagged this way. These include: Partition calculus, diamond principles, square principles, …
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For questions about type theory, including normalization, dependent types, identity types, inductive types, universe types, functional programming languages, proofs as programs in simply-typed lambda-…
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For questions about dividing groups of objects based on their properties.
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For questions about Hopf algebras and related concepts, such as quantum groups.
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for concepts related to Hölder continuity (a generalisation of Lipschitz continuity) and the related Hölder spaces.
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Questions on doubly periodic functions on the complex plane such as Jacobi and Weierstrass elliptic functions.
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for the error and complementary error functions (erf and erfc). These are special functions formed by taking definite integrals of the Gaussian/normal distribution function.
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defined block wise, like $\pmatrix{A&B\\ C&D}$ where $A,B,C$ and $D$ are themselves matrices. Use this tag with (matrices), and often with (linear-algebra).
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for questions about standard, cultural practices in mathematics.
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for questions related to absolute convergence a series.
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For questions about approximating real numbers by rational numbers.
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Various aspects of writing mathematics such as style, notation, grammar, frequently used phrases and common mistakes.
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A nilpotent matrix has $A^n=0$ for some integer $n$.
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In mathematics, a monoidal category (or tensor category) is a category C equipped with a bifunctor ⊗ : C × C → C which is associative up to a natural isomorphism, and an object I which is both a lef…
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a transformation that fixes a line or plane or a more general subset. Reflections appear in geometry, linear algebra, complex analysis, differential equations, etc -- therefore, this ta…
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a construction in group theory generalizing the direct product. It arises as the structure of a group $G$ with a normal subgroup $N$ having a complement $N$.
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For questions on solvable groups, their properties, and structure.
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Questions on elliptic integrals, integrals that involve the square root of a cubic or quartic polynomial.
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For questions about networks that inhibit source and sink nodes and a notion of flow.
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Optimal control theory, an extension of the calculus of variations, is a mathematical optimization method for deriving control policies. (Def: http://en.m.wikipedia.org/wiki/Optimal_control)
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For questions about infinitesimals, both in an intuitive sense as well as more rigorous settings (see also [nonstandard-analysis]).
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For questions about groups where the underlying set has infinite cardinality.
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In mathematics, in particular abstract algebra, a graded ring is a ring that is a direct sum of abelian groups $R_i$ such that $R_i R_j \subset R_{i+j}$. (Def: http://en.m.wikipedia.org/wiki/Graded_ri…
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Questions related to arithmetic progressions (sequences with a constant difference between the terms)
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For questions on Catalan numbers, a sequence of natural numbers that occur in various counting problems.
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Questions on Bernoulli numbers, a special sequence of rational numbers that arise as the coefficients in the power series expansions of certain elementary functions.
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curve in the complex extending from a branch point of the function.
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For questions about mathematical constants, that are "significantly interesting in some way".
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called homogeneous if the following condition is satisfied: If $\phi(x)$ is a solution, so is $c \phi(x)$, where c is an arbitrary (non-zero) constant. (Def: http:…
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In mathematics and theoretical physics, the term quantum group denotes various kinds of noncommutative algebra with additional structure. (Def: http://en.m.wikipedia.org/wiki/Quantum_group)
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Questions on the Riemann hypothesis, a conjecture on the behavior of the complex zeros of the Riemann $\zeta$ function. You might want to add the tag [riemann-zeta] to your question as well.