Seeking the shadowy definitions of the OSR I have followed the dusty trail from Tenkar's Tavern to The Chronicles of Ganth. And so....
A funhouse. A nightmare. A dreamscape. An ever changing endless twisting of tunnels, chambers, caverns and worlds within worlds ruled by all manner of creatures, denizens, cults, sorcerers, races, demons leading deeper beneath the surface or out into cosmic realms or into the depths of a soul or just out into a back alley. In other words anything can happen and real world rules don't apply. When you walk in be prepared for anything. ANYTHING!!
Railroad: Start of an adventure; point A. End of an adventure; point Z. Don't worry, your not going to die while traveling from B through Y. Stay off the grass!
OSR: A revival in the interest and workings of the origins of the loose play-style of out of print Role-Play gaming rules putting good times, friends, adventure, carousing, gratuitous violence, raunchy jokes and beer above rule-lawyering and character builds.
Crunch & Fluff: The ratio between complicated rules and dice rolling involving charts and bell curves and story padding with the sweet-spot being whatever a group prefers. (Me, I gave up crunch back in 1982 with Champions and Top Secret. Never looked back).
Multi-classing: Face it, every character is multi-classing; you're a thief and a grave-robber with some other skills such as wielding a blade or casting a spell.
Clones, Retros, Sims, etc etc etc: Interchangeable terms for DIY rules and house-rules that are made public in creation of or support of the OSR (see above).
Module/Adventure: An interchangeable aspect of a campaign setting that can be inserted at any time and at any place during an ongoing adventure. Can be customized as needed to compliment the mood or story presented in the campaign.
Hit Dice: Type and amount of dice that determine how much damage a character can take before he/she/it dies, usually from a horrible death, rendered meaningless if bit by a spider, centipede or other poisonous creature or after having consumed the wrong liquid.
Sandbox: An anything goes campaign setting where player actions determine the fate of kingdoms, destinies of characters, explorations of hidden lands but usually means getting drunk and setting fire to a tavern. No planning required.