Swordfish
A Swordfish is the three-line version of an X-Wing. Pick a digit and look for three rows (or columns) where that digit's candidates are confined to the same three columns (or rows). The nine intersection cells form the pattern, and because each of the three lines must contain the digit once, the digit has to occupy three of those cells — one per line. That locks it out of every other cell in the three shared lines.
What is a Swordfish?
A Swordfish is a single-digit fish pattern that spans three rows and three columns at the same time. The setup is: in three different rows, a particular digit can only go in cells that fall within the same three columns. The nine cells where those rows and columns intersect form a 3×3 grid — the "swordfish". Because each of the three rows has to place the digit once, and the only available spots are inside that grid, the digit must land on three cells — one per row, and necessarily one per column (otherwise a column would get two copies, which is impossible). So each of the three columns receives exactly one digit from the Swordfish, and every other candidate for that digit in those three columns can be erased. The same logic works with rows and columns swapped.
How to spot it
Swordfish are harder to find than X-Wings because you're tracking three lines at once instead of two. The reliable way in is the same digit-first scan: pick a candidate digit and, for each row, note which columns it can land in. If three rows each restrict that digit to at most three columns, and those column-sets all draw from the same three columns, you've found a Swordfish. The cells don't have to be filled in every intersection — what matters is that the digit's candidates in those rows stay within the three shared columns. The same scan works column-first. Swordfish are rare and tend to appear only in the hardest puzzles.
The logic
Every row and every column places each digit 1–9 exactly once. If a digit is confined to three columns in each of three rows, and those columns are the same three across all three rows, then the three rows must place the digit in three of the nine intersection cells — one per row. The three placed digits necessarily fall in three different columns (otherwise some column would get two copies, which is impossible). So each of the three columns receives exactly one digit from the Swordfish, and no other cell in those columns can hold it. The same argument with two rows gives an X-Wing, and with four, a Jellyfish.