An acronym that stands for Truly Laughing Out Loud. Created because the popular "LOL" and "haha" have recently evolved into a way of acknowledging what someone says to you on AIM. TLOL indicates that you are, in fact, laughing out loud.
"I just got back from the movies"
"lol cool" (NOT actually laughing out loud)
"...then I stepped on the cat's tail"
"omg tlol" (TRULY laughing out loud)