TL;DR - Humans are not limited to Terra, but one from Terra is called a "Terran".
Even within the comics, Humans are not limited to Earth - variations of the race are scattered throughout the universe. The Homo Sapiens race was originally created by the Celestials, a space-faring race easily comparable to gods.
We know that Celestials exist in the MCU, as Knowhere was still shown to be the skull of a Celestial (even though the race wasn't named as such on-screen). Therefore, it stands to reason that the Celestials may still be the creators of the human race, and could have spread their experiment throughout the galaxy. In fact, The Collector himself is one of the oldest beings known to exist and appears very similar to a human in appearance. We know he isn't one due to his nearly-immortal lifespan, but it's quite feasible to theorize that both races have the same origins.
Also, the fact that Peter Jason Quill is stated to be half-human indicates that humans share enough DNA with other races to produce viable offspring. This is further substantiated by the fact that Nova Corps officer Dey is shown to have a half-human daughter with his Xandarian wife.