The outer seam (where the fabric comes together) of a pair of pants. The outseam is used to refer to the measurement of the length from the waist all the way down to the cuff.
The outseam is commonly what people think about when they think of the length of a pair of pants. However, in most cases, the actual length displayed on pants in stores is the inseam, rather than the outseam, so that length should be used when sizing a new pair of pants.
The outseam is typically only used when pants are being made to measure by a tailor.