What is it called when your l5 is fused to your pelvic bone...

It is known as a : Transitional vertebra and some suspect that with certain configurations it may predispose one to back problems by altering the normal mechanics of the spine and this is known as bertolotti's syndrome

Fusion: That is called "fusion", although technical terms for surgical fusion are "spondylodesis" or "spondylosyndesis". Fusion may be unrelated to surgery, as some bones may be fused congenitally (person is born that way).