doorwave

n.
a type of greeting or farewell typically given and received by co-workers in office settings. The person receiving the doorwave is inside his/her office and the person giving the doorwave is walking past the door to said office. The person giving the doorwave does not slow down, but instead extends the hand in front so that it is visible to the office occupant through the doorway. This hand is maintained at the same position relative to the doorframe until the giver of the wave has walked so far that it is impossible to do so.

This type of wave is often given when the giver does not have time to give a more formal greeting or farewell.