A timehead was a hypothetical feature that Amy Pond suggested to the Eleventh Doctor when he questioned why she told him she was pregnant rather than her husband Rory Williams. She explained her fear that travelling in the TARDIS while pregnant would have some sort of effect on the baby, suggesting it could have three heads "or, like, a timehead or something." The Doctor then asked her what a timehead was, to which she revealed she had no idea. They both proceeded to laugh at the suggestion. (TV: Day of the Moon)
While Amy's child did not have a timehead, she was indeed affected as a result of being conceived on the TARDIS while it was in flight in the Time Vortex. While human, she was born with Time Lord DNA. The Doctor, having discovered the baby's Time Lord DNA, recounted Amy worrying that her baby would have a timehead on Demon's Run while trying to make sense of it all, leading Vastra to note that only he would ignore the instincts of a mother. (TV: A Good Man Goes to War)