Lamination
Lamination is the repeated folding and rolling of dough around a layer of butter, producing alternating layers of dough and fat. In the oven the water in the dough turns to steam, forcing the layers apart, while the butter keeps them from fusing — that is what gives a croissant and puff pastry their structure.
The layer count grows geometrically: each single turn triples it, each double turn quadruples it. Classic puff pastry is usually taken to several hundred layers; croissant dough to noticeably fewer, because a yeasted dough needs room to rise as well.
The condition for success is that dough and butter have the same plasticity. Butter colder and harder than the dough shatters and tears the layers; butter warmer than the dough is absorbed into it and the lamination disappears. Hence the rest in the fridge between turns, and working quickly in a cool room.