Definition
An inseam is the seam running along the inside of a trouser leg from the crotch to the hem; the word also names that length as a measurement.
On a trouser pattern, the inseam joins the front and back leg pieces along the inner leg. It meets the crotch seam at the crotch point, where four seams cross, and its length must match on the front and back pieces so the legs do not twist.
As a body measurement, the inseam, called inside leg in the UK, runs from the crotch to the floor or to wherever you want the hem to fall. There are two reliable ways to take it: measure the inside leg seam of a pair of trousers that fit you well, from the crotch seam to the hem, or stand straight without shoes and measure from the top of the inner thigh to the floor. It is the most direct way to set trouser length, and the one used in jeans sizes such as W30 L32, where L is the inseam in inches.
When sewing, some patterns have you sew each leg's inseam first and then join the legs along the crotch curve; others sew the crotch first and the inseam afterward in one continuous pass from hem to hem. Follow the order your pattern gives, and match the notches at knee level so the legs hang straight.
Related measurements: outseam, rise and crotch curve.