One or more characters are "standing"; their bodies are being supported by their feet. (or by either foot)
Most often, standing characters are absolutely upright; search for standing bent_over or standing leaning_forward for when they are not.
Some tags are subsets of this:
This tag is still applicable where the character is in the middle of certain actions that keep upright posture such as walking.
The following tags implicate this tag: standing_sex, pantyshot_(standing), standing_on_liquid, and standing_on_one_leg (learn more).