Delta is sometimes used in an engineering context to describe a change of something. For example, a software versioning system tracks changes to source code files and refers to the changes as “delta”.
I find it useful to refer to changes of state in sound the same way. The organization of delta in sound affects the listener’s interest level and experience. Too much delta, and it’s too hard to keep up. Not enough delta, and things are hard to pay attention to.
There are some basic, common delta vectors (think of a vector as simply a line). One vector describes acceleration/deceleration and the other describes expansion/contraction.
The art of organizing sound involves the management of change in listening objects. The science of sound provides some guidelines for managing this change, for managing delta.