A cameo commonly refers to a small (and usually unexpected) role in a work of fiction performed by a person of prominence, be it a celebrity, a well-known individual, or a person who holds significance to the specific work at hand. Cameos are brief and lack any weight in the story, so their noteworthiness comes solely from the inherent importance of the person playing the part.
In a medium like comic books, which doesn't involve theatrical performances, the principle behind a cameo translates to the depiction of a person of prominence in a small role within a story. Due to the nature of comics, it's not uncommon for cameos to involve the self-insertion of a story's creator(s). Marvel staff members, most notably Stan Lee and Jack Kirby, have made plenty of cameos in comic books, sometimes in the form of self-insertions and later through homages from later creators.
The concept of cameos also applies to fictional characters within a shared universe, like it is the Marvel Universe. In this case, a cameo consists of the minuscule appearance of an established character in the story of a comic book series outside their normal sphere of influence. This appearance is unexpected, inconsequential to the story, and only serves the purpose to amuse readers.
An example of a cameo from Tony Stark: Iron Man #3 sees a scene take place in the Uncanny Valley, a secret bar for robots and androids. While most of the patrons on the establishing shot are unidentified generic robots, one of the customers is H.E.R.B.I.E., the robotic assistant of the Fantastic Four.
[top] [Edit Cameo]