X-Wing
A X-Wing is a size-2 basic fish for one digit. In one orientation, the digit’s candidates in 2 base rows are confined to 2 cover columns; the transposed orientation swaps rows and columns. The digit can be removed from the cover sectors outside the base sectors.
What is an X-Wing?
The size counts 2 required placements, 2 bases, and 2 covers—not a requirement for all 4 intersections to contain candidates. Each base contributes at least one candidate, and every relevant base candidate lies in the selected covers.
How to spot it
Choose one digit and select 2 rows whose candidates are confined to the same 2 columns, or 2 columns confined to the same 2 rows. Sparse intersections are valid. Then inspect the cover sectors outside the selected bases.
The logic
Each of the 2 base sectors must place the digit, and all 2 placements lie within the 2 cover sectors. Those placements occupy the covers, so any other occurrence of the digit in a cover but outside the bases can be removed. Size-one pointing or claiming is smaller than an X-Wing.