I've seen the term AFOL around; for example, this site has an afol tag. What does AFOL stand for?
AFOL stands for "Adult Fan Of LEGO".
From an unofficial LEGO acronym glossary:
Adult Fan of LEGO. The most common term used to refer to LEGO fans who are adults. Generally pronounced to rhyme with “hay-foal”.
Or from Brickipedia's glossary (also unofficial):
AFOL - Adult Fan Of LEGO