My thumb only has one joint. what is this called?

It can be a variant : It is hard without a picture or x-ray. There is a variant of the thumb called triphalangeal thumb where the thumb has 3 phalanges (like a finger) instead of the two phalanges that a "norma;" thumb has. There are conditions where the thumb is hypoplastic. This means it is underdeveloped compared to a "normal" thumb and may look thinner, have a different ligament structure and have thinner muscles at its base there are conditions known as arthrogryposis where joints do not develop normally and stiff fingers can result. While many arthrogrypotic limbs are not limited to the fingers alsone there are some mild variants known as distal arthrogryposis where only the finger joints are affected hand differences or hand congenital ( born with) anomalies are common. Some hand differences are beyond classification ( in other words we don't have a name for them they just are what they are) the good thing is that for most with a congenital thumb difference or even a stiff thumb joint that occurs due to an injury, it is not a problem doing everything and it is not a limiting factor in any type of job, hobby or activity you might want to search "congenital hand differences" on the web to read more.