The term delta refers to the difference between two values or states. The name originates from the Greek letter Δ (delta), which is equivalent to the letter D in the Roman alphabet. Delta simply refers to the use of the letter Δ as a shorthand for difference.
The term delta is commonly used when communicating changes in speed, position, or acceleration of a physical or virtual object. It's also used when describing changes in the volume or frequency of sound waves.
For example, when describing how far an object on the screen moves left-to-right, one might use the term delta x or Δx.
Likewise, given the new value of X and its old value, you might compute the delta like this:
let deltaX = newX - oldX;
More commonly, you receive the delta and use it to update a saved previous condition:
let newX = oldX + deltaX;