What Is Delta in Math?

Although it usually refers to change, delta itself is a Greek letter that can also be used as a variable in equations. Typically, if used to signify change, it is followed by another variable, such as ?X.

Deltas are written in both upper and lower case. Upper-case deltas look like triangles (?), and lower-case deltas look like a stylized d (?). When using deltas as the syntax for writing differential equations, the lower-case delta is more common.