Grayson in K-theory handbook: page 60. Definition and some properties, also multirelative K-theory with supports.
Defined for a scheme together with a list of closed subschemes (in Grayson).
Used (by Bloch and Lichtenbaum, I think) to construct the motivic spectral sequence for the spectrum of a field. See Grayson, ref above.
arXiv: Experimental full text search
KT (K-theory), AG (Algebraic geometry)
nLab page on Multirelative K-theory