Seed word
A seed word is the word a puzzle is built around: the constructor places it first and fits every other word to it.
In crossword construction, the seed is a lively long entry the constructor wants in the grid badly enough to design around — a fresh phrase, a name nobody has used yet. It goes in before the black squares are settled, and the rest of the fill is chosen to interlock with it. In word searches and criss-cross grids, the seed is usually just the longest word in the list, placed first because it has the fewest positions that fit.
The word carries a second, unrelated sense in software. A seed is the starting value handed to a random-number generator, and the same seed always produces the same sequence. Daily puzzles use the date as a seed, which is how everyone gets an identical puzzle on the same day without that puzzle ever being written down and stored — the generator rebuilds it from the date on demand.
The two senses share a metaphor and nothing else. Context normally makes it obvious which one is meant: a constructor seeds a grid, a program seeds a generator.
Word Search Generator — places your longest word first, then fits the rest around it.