The triple bar, ≡, is a symbol with multiple, context-dependent meanings. It has the appearance of a "=" sign with a third line. The triple bar character in Unicode as codepoint U+2261≡identical to (HTML: ≡≡). LaTeX\equiv corresponds to the triple bar.
In logic, it has a similar meaning to the if and only if connective, ⇔. However, in some texts ⇔ is used as a symbol in logic formulas, while ≡ is for reasoning about those formulas (as in metalogic).
In mathematics, it is sometimes used a symbol for congruence (although not the only one). Particularly, in number theory, it has the meaning of modular congruence: if N divides a − b.
This symbol is also used when it appears in an equation which is a definition of its left-hand side, that is an equation which is not derived but instead defined.
It is also used for "identical equality" of functions; one writes for two functions f, g if we have for all x.