Copy Martial Arts are a minor school of martial arts that only appears in the anime continuity of Ranma 1/2. Debuting in the episode Enter Ken and His Copycat Kerchief, they are the signature fighting style of the episode's titular villain, Copycat Ken.
Little is known about this style - in fact, it's not even named as a style outside of the original Japanese language dialogue of the episode. By taking pictures of others, a practitioner of this style can then assume their form by temporarily obscuring their form with a cloth. In a copied form, a practitioner of Copy Martial Arts finds their physical abilities and skill level set to the equal of the person whose form they are copying. This often guarantees victory; many martial artists underestimate the "Copycat" as somehow weaker than themselves, and are surprised to find themselves facing off against a perfectly matched opponent.
Copycats can also shift between multiple forms, allowing them to fluidly adapt their combat style to their situation by using whichever identity is most advantageous to the moment. However, if a copycat attempts to shift between forms too rapidly, they may start getting "mixed up", and be unable to stop changing shapes.
There is no seeming limit to who a copycat can mimic; Copycat Ken was able to freely assume the forms of not only Ranma Saotome and his teenage rivals (Tatewaki Kuno, Ryoga Hibiki and Mousse), but also the much smaller form of Happosai and even switch genders by taking on the forms of Akane Tendo and Ranma's own female side.
Despite its formidable abilities, Copy Martial Arts does have some weaknesses. Firstly, a practitioner can only access copied moves when using the form of a person they were copied from; Copycat Ken is unable to use Ranma's Kachu Tenshin Amaguriken or Ryoga's Bakusai Tenketsu unless he is in their forms. It's also possible, but not confirmed, that a copycat may only be able to use techniques they have photographed, and thusly can be caught off-guard if the person they are copying reveals a technique that the copycat didn't observe and capture beforehand. Finally, a copycat's "mirrored" physical abilities and skill are set to the level that the person they are copying had at the time they were photographed; by managing to hone their prowess to a higher level, the original can "outdo" the copycat and defeat them by weight of skill. This is how Ranma is able to defeat Copycat Ken; by trusting to his (currently her) instincts, he managed to push on to a new level of speed and power with his Amaguriken technique, becoming too fast and accurate for Copycat Ken to parry.
How this school is possible remains a mystery. Magic may possibly be involved.