Definition: Sinter (verb transitive) is to bring about the agglomeration (stick-togetherness) of certain materials through heating. In ceramics, the clay particles sinter even before they actually begin to melt into a glassy state (vitrification).
The purpose of bisque firing is to reach the clay's sintering temperature, without heating it to the point of vitrification. Sintered ceramic material is still quite porous; therefore, raw glaze coats adhere much better.
Examples:
The pots sinter in the kiln, beginning at about 1650⁰F (900⁰C). (verb)
The kiln's firing was interrupted and the pots are not sintered, so cannot be glazed. (adjective)