There are 4 distinct cycles.

1- Seed - germinated seed with taproot is still a seed.
2- Seedling - this is once the seed has opened up and has shed the seed husk and opened up the first set of leaves called Cotyledon leaves. They will stay in the seedling cycle until you have your 4th set of true leaves formed. (The Cotyledon leaves don't count)

3- Vegetative cycle - You enter the veg cycle once you have 4 sets of true leaves.

4 - Flower cycle - the light in 95% of the strains triggers this once you switch to 12/12. It does take about 3-5 day's for the plant to change modes so to speak.