Not having any distinguishing marks or features. The unmarked package was left on the doorstep, raising suspicion among the neighbors.
Not indicated or highlighted in any way, such as on a map or chart. The unmarked trail led us deeper into the forest, away from the main path.
Not being guarded or closely watched by an opponent, especially in team sports. The player was left unmarked in the penalty area, allowing him to score easily.
Being the neutral, default, or conventional member of a contrastive pair or set in linguistics. In English, the plural noun without a suffix, like “sheep”, can be considered the unmarked form.