Question

What's the definition of streak

Answer 1

Generally, the old fashioned term I'm familiar with is to "Walk or run while naked
" This is usually thought of as being naked in public, but it can also be what you do after a long day of work when you get home.

Answer 2

In terms of Snapchat streaks...
This is a number of days two users have been sending a picture or video to each other.

In the general term of a streak...
An irregular line left from smearing or a continuous series of like events.

It all depends on the context.