What does the triangle equals sign mean?

[pellman]

Looks like $$\triangleq$$. I first came across it on the wikipedia article for Lie derivative. http://en.wikipedia.org/wiki/Lie_derivative#The_Lie_derivative_of_a_function

 

[LCKurtz]

It is sometimes used to mean "is defined as" or equals by definition.

 

[HallsofIvy]

Agree with LCKurtz.

 

[pellman]

Gracias, amigos.

does it differ in meaning from the triple-equals sign? $$\equiv$$

 

[elibj123]

The triple-equals is usually used to denote that a function identically equals some constant number (or another function).