An Ally or Straight Ally is a cisgender, heterosexual/romantic, allosexual/romantic (cishet) individual who is not part of the LGBTA+ community but supports the community. An ally acknowledges that LGBT people face discrimination and thus are socially disadvantaged, supports LGBT+ rights, and challenges homophobia, transphobia, etc. Not everyone who meets this definition identifies specifically as an "ally". Not all cishet people are allies.
The A in LGBTA+ does not stand for ally. Rather it stands for asexual, aromantic, and agender.