\triangleqThe symbol \triangleq is used to denote a definition or an equivalence by definition. It is often read as 'is defined as' or 'is equivalent to by definition'. This symbol is used in mathematical expressions to explicitly state that the left-hand side is defined to be the right-hand side.
Defines 'a' to be equivalent to 'b' by definition.
a \triangleq bDefines the function f(x) as x squared plus one.
f(x) \triangleq x^2 + 1Defines energy E as equivalent to mass m times the speed of light squared, according to the famous equation by Einstein.
E \triangleq mc^2