Builder gel is the structural member of the gel family: a thick, high-viscosity gel used to add architecture rather than colour. Techs use it to reinforce natural nails (as an overlay), to build an apex — the subtle structural hump that gives a nail its strength — and to sculpt extensions over forms. It cures rock-solid under a lamp but keeps slightly more flex than acrylic, which is why builder sets tend to bend before they snap.
It comes in pots (hard gel, applied by brush) and in bottles — the famous “builder in a bottle,” or BIAB. Soft builder gels soak off in acetone; true hard gels must be filed off, a distinction worth asking about before your first appointment, because it changes what removal looks like three weeks later.