What is the proper term for the "female" section of the piece that the "studs" insert into? I have heard "pipes", "tubes", "underbits". Is there an actual term for the female section?
Chapter 1, Page 4, The Unofficial LEGO Builder's Guide:
The tube is the other half of the mechanism that helps bricks stick together. Tubes capture the studs so that you can join LEGO elements and know they won't fall apart.
Bricklink (at the time of writing) has 5 catalog items using the term 'stud receptacle'.
Members of the French club FreeLUG refers to these as antistuds, and there is at least one LDraw part that uses that same naming. Considering a lot of other parts don't, however, it's probably not widely accepted. And it's not valid at Scrabble either.
You could also use mortise by analogy with the woodworking assembly (a stud is also usually called tenon in French), but again, I've never heard it in LEGO circles. Slot might be appropriate too.