When cloth is made on a loom, two sets of threads cross each other. The threads held tight, running the long way along the cloth, are the **warp**. The threads carried side to side, over and under the warp, are the **weft**. So the weft is the crossing thread — the one that actually travels back and forth as the weaving happens. This is a technical word from weaving and textiles, and in that world it is exact and everyday: a weaver, a fabric maker, or a textile student uses it without thinking. Outside that world, most English speakers only meet it in the fixed pair **"warp and weft."** That phrase has become a way of describing the basic criss-crossing structure of anything — "the warp and weft of village life" means the threads that make the whole thing hold together. When you meet the word in a novel or an essay, it is almost always doing that figurative job. Grammar notes: it is a noun, usually uncountable or treated as a single set ("the weft"), and it takes `the` far more often than `a`. In some places, especially older British usage, you may also hear `woof` for the same thing — same meaning, different word, and unfortunately identical in spelling to the sound a dog makes. `Filling` is the common American factory term.