Someone who modifies their body in a way that turns them from a biological being called a homo sapien sapien into something, presumably better, and technically immortal.
This does not include prosthetics for people after accidents who only want to experience a somewhat normal mortal life.
You implant computer parts into your brain to calculate mathematical problems faster. You are now Transhumanist.
You inject yourself with synthetic blood components that include stronger immunology, better termination of rogue cells and molecules, better gas transport. You are now a Transhumanist.