A KC-135 is a Tau'ri airplane used on Earth for refueling other planes such as F-15 fighters. It is a variant of the DC-10 passenger aircraft.
In an alternate timeline created by Ba'al, a KC-10 refueled SG-1's F-15s. It was destroyed by a Ha'tak's staff cannon blast when Qetesh ordered Ba'al's fleet to bombard Earth. (SG1: "Continuum")