1. Diversity and inclusion - LGBTQ+.
In the context of diversity and inclusion and LGBTQ+ rights, an ally usually means a person who does not identify as LGBTQ+, but supports the rights of people who do.
An ally can also refer to a person who belongs to one group within LGBTQ+, and supports the rights of people in other groups.
More generally, an ally is anyone who supports the rights of a group that is disadvantaged, without themselves being a member of the disadvantaged group.
For example, a man who supports women's rights.