A term in art referring to a collage made with three dimensional objects. While a collage may be slightly three dimensional, as the paper, and other things that make it up are three dimensional, an assemblage is made of three dimensional materials. Assemblage is different from sculpture in that it tends to be done on a relatively flat surface - it is something that could (perhaps) be hung on a wall, whereas sculpture is more often placed on a horizontal surface.
The difference between sculpture and assemblage: assemblage tends to have a two dimensional image plane; sculpture tends to have a three dimensional image plane - there is some value in the depth of the work.
Disclaimer. These are general terms. Different artists may see things differently. This is a reasonable representation of the popular use of the terms.