Related terms include cishet, oppression, erasure.
"As an ally to the LGBT+ community, I have chosen not to marry my opposite-sex partner until same-sex marriage is legalized."
Within the context of progressive culture, an ally is a person who is not a member of an oppressed group but who supports civil rights and social movements associated with that group. An ally acknowledges his or her position of relative privilege and uses that position to create change within the larger culture and society. Examples of acts of allyship include a sighted person writing image descriptions for a website, a cisgender man marching in a pro-choice rally, or a white person boycotting a business known for racial profiling.
The role of an ally within an oppressed community can be a point of some controversy. For example, Mia McKenzie of Black Girl Dangerous wrote a widely-circulated article criticizing self-identified allies for putting themselves in the spotlight at the expense of the people they claim to support, saying allyship is "not supposed to be about you. It's not supposed to be about your feelings. [...] It's supposed to be a way of living your life that doesn't reinforce the same oppressive behaviors you're claiming the be against."
The general consensus within oppressed communities is that a role of an ally is to listen to feedback and take their role seriously and humbly, never choosing to speak for, or over, oppressed minorities.
The term "ally" is much older than many terms used in the progressive community, first used in progressive culture with the birth of PFLAG, or Parents, Families, and Friends of Lesbians and Gays, in 1973.