Delta refers to change in mathematical calculations. In some cases, this means a difference between two values, such as two points on a line. In other cases, it refers to the rate of change, such as in a derivative.
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.