Well, its mostly an indoor term.
Stretch Phase = put a vegging clone in flower, if its indica it will double in size, if its sativa it can nearly triple.
Stretch phase is day 0 of flower till about week and a half to two weeks in.
It takes that long for the buds to set in and start growing. So the plants like to have nitrogen during that time.
See the first two weeks of flower the plant is getting ready to flower but its not flowering yet, its stretching and getting ready to.
When you see flowers start on your growing tips that means you have already been in flower for at least a week, you have been in the stretch phase.
For my location it starts around aug 1st.