What does a Celtic knot tattoo symbolize?

each knot pattern has a different meaning. You'd really have to research a specific design, or at least ask a question about a specific knot, to get a proper answer. IMO tattoos mean specific things to the person wearing them, and sometimes they are just for decoration. there isn't really "symbolism" in the tattoo world these days. people just get things tattooed on them because they think it looks cool... but some people do get tattoos that mean something to them, and most times it's not that apparent to people looking at them.