In math, the word “delta” stands for “change.” In some situations, this means the difference between two points on a line. In other situations, like a derivative, it means the rate of change.
Even though delta usually means “change,” it is also a Greek letter that can be used in equations as a variable. If it’s used to show change, it’s usually followed by another variable, like?X.
Deltas are written with both capital letters and small letters. Deltas in uppercase look like triangles, and deltas in lowercase look like a stylized d. When writing differential equations with deltas as the syntax, the delta with a lowercase letter is more common.