50/50:
A fabric blend of cotton and polyester. These are the best kinda shirts if you like shirts that thin over time. Modern 50/50 shirts are extremely hard to come by. It's important to note that not all vintage shirts are 50/50 blends. Some vintage was made with 100% cotton or other fabric blends.

Bootleg:
Generally in reference to t-shirts, this means the shirt was unlicensed. A lot of vintage band shirts were made or printed in small batches and sold at concerts. Bootleg can mean the t-shirt might be more valuable, even, as it wasn't mass produced.

Deadstock:
See "NOS" below.

Mounted collar:
See "Ringer Tee" below

NOS:
Meaning "new old stock", also commonly referred to as "deadstock". These are vintage items that are unused. Think of it as the unworn version of vintage overstock products. The small difference between NOS & deadstock is that in some instances, deadstock items can be found with wear from age or storage. 

OOAK:
"One of a kind"

Raglan:
A sleeve style most commonly seen on classic baseball t-shirts. This is a sleeve stitch line that goes from the underarm to the collar, as opposed to the top of the shoulder. Most vintage raglan sleeves feature a contrasting fabric color to the main fabric.

Ringer tee:
T-shirts or tank tops with collars/sleeve hems that are "mounted" onto the main fabric. Generally a thicker knit hem, often in a contrasting color to the main fabric.

Single-stitched:
A single stitch line on hems to make the edges not raw and fray (generally on t-shirts). Most new shirts (1990s to current) have double-stitched hems. This is a good telltale sign when determining if a piece is vintage or not, however not every vintage t-shirt is single stitched.