Definitions of sitting:

• noun:   the act of assuming or maintaining a seated position

  Example: "He read the mystery at one sitting"

• noun:   (photography) the act of assuming a certain position (as for a photograph or portrait)

  Example: "He wanted his portrait painted but couldn't spare time for the sitting"

• noun:   a session as of a legislature or court
• noun:   a meeting of spiritualists
• adjective:   not moving and therefore easy to attack

  Example: "A sitting target"

• adjective:   (of persons) having the torso erect and legs bent with the body supported on the buttocks