Inseam refers to the seam in a trouser/pants or short that runs from the crotch to the hem. Leg length is measured based on the inseam. Most often refers to the inside seam of pants.
It also refers to the measurement from the pant crotch down to where the pant leg falls on the shoe; the inner seam on the legs of a pair of pants.
The Glossary of Fashion covers everything you need to know about the world of fashion