Jack is the Tekken character name used for a series of mass-produced combat robots.
The Jack line consists of powerful humanoid machines developed by organizations including the Mishima Zaibatsu and G Corporation. Different models appear across the series, including the original Jack, Jack-2, Gun Jack, Jack-4, Jack-5, Jack-6, Jack-7, and Jack-8. Their main strengths are enormous physical power, durability, and heavy-hitting attacks.
The models are not all identical in purpose or personality. Jack-4, for example, was mass-produced by G Corporation as a disposable military force and was sent to assassinate Kazuya Mishima. Jack-5 was created by Jane, a scientist associated with the Jack project, and entered the fifth King of Iron Fist Tournament.
Combot is another robotic fighter, but it is a single experimental machine associated with the Mishima Zaibatsu rather than the name of the mass-produced Jack series. Mokujin is a wooden training dummy brought to life, while Kuma is a bear. “Jack” is therefore the canonical answer.