[ben-schaaf]

Delta-v is defined as "change in velocity", ie. how much a spacecraft can change its velocity. I think you're talking about relative velocity.

[tyre]

I believe GP was referring to delta as in mathematics, which refers to difference and not necessarily change. So the delta between +7km/s and -7km/s is 14km/s.

You are technically correct, though based on common usage patio11 got their point across.

[patio11]

Yep; appears I misremembered a long-ago physics class.