What is the term for "abnormal curvature of fingers"?

It depends: There are many, it all depends on the cause and type of curvature.

Curved fingers can b: Congenital: clinodactyly, camptodactyly, distal arthrogryposis, mild symbrachydactly aquired: fracture, dislocation, tendon injury degenerative: arthritis ( many subtypes) developed: such as dupuytren's disease. Its a long list and depends upon age, history and exam.