Lindens or L$ (sometimes just abbreviated L) - virtual currency for Second Life. Can be earned in-world with quests, working a job, and other ways. Can also be purchased directly from https://secondlife.com/my/lindex/buy.php for real money, or sold back for cash, based on an exchange rate.
Landmark or LM - a place mark to teleport to a different location.
Notecard or NC - an in-world object that is a text-only notecard. Often used for providing information about purchases or sims. You can also leave notecards for other users, either directly given to them or deposited in designated mailboxes.
Mouselook or ML - focus where the camera is in first-person view, and the mouse controls where you look. It is possible to force someone into Mouselook using RLV. This mode restricts what you can click on in-world, and can block access to HUDs and the use of objects. You also cannot look at yourself in this mode.