An XCross, short for extended cross, sometimes referred to as Cross+1, is an intuitive sub-step that creates the cross and the first F2L pair simultaneously. The idea of a 2x2x2 and the cross combined as two separate phases was first proposed by Anthony Snyder in the 80's. In July 2003 Chris Hardwick proposed the full XCross as a single phase to be planned during inspection.
XCross is relatable to the Petrus Method due to it starting off with a similar step.
If multiple F2L pairs are done at once, it is written as XXCross, XXXCross, or XXXXCross depending on how many slots were solved during the phase. It is rare for an XXCross to occur, XXXCross is even rarer, and XXXXCross typically only occurs in poorly scrambled solves.
Although sometimes used interchangeably, XCross and Cross+1 refer to different concepts. Both, however, result in the same state.
If this state is reached by first solving a Cross and then adding the F2L pair(s), this is referred to as a Cross+1, Cross+2, Cross+3 or Cross+4, depending on the number of slots solved. The difference between just solving a Cross followed by the pairs and a Cross+1 is that the pairs are usually planned in inspection.
Although the movecount of Cross+1 is usually higher than XCross, this is compensated by the better ergonomics, since blockbuilding often isn't very fingertrick friendly.