1. A well thought out, and designed project using the finest parts, and workmanship. 2. A cobbled together piece of junk using what trash you could get for nothing. Custom is in the eye or the stupidity of the beholder.
You can't polish a turd, but you can spray paint it gold, and call it custom.
A high performance car that has been modified visually and mechanically to gain attention. Usually an import car that has been lowered, painted, had a spoiler kit added and engine tuned significantly - usually turbo'ed.