Simple Definition:

Delta in math is a symbol that represents the difference or change between two values. [Symbol: a character or image used to represent something]

For example, if the temperature yesterday was 80 degrees Fahrenheit and today it is 85 degrees Fahrenheit, the delta temperature is 5 degrees Fahrenheit.

Very Simple Definition:

Delta in math means the difference between two things.

For example, if now you have 5 apples and later you get 3 more apples, the delta (difference) is 3.