Straight is a slang term for heterosexual, which is a sexual or romantic attraction to people of the 'opposite' gender and is considered the opposite of gay. Unlike lesbian, there is no gender-specific terms, 'straight' is used for both straight men and women.
While straight is often used to describe non-LGBT people it is possible for straight people to be part of the LGBT community if they are not cisgender. Furthermore, someone could be asexual and heteroromantic, or aromantic and heterosexual, with also makes them LGBT.
Another definition of straight is someone who is attracted to those of a different gender, as using the word 'opposite' implies there that are only two genders. Straight usually means men attracted to women and women attracted to men. A relationship with two people of different genders caused by one of the parties being non-binary can be called diamoric.
Etymology
The prefix hetero- comes from the Greek word heteros, meaning 'different'. The term "heterosexual" was coined shortly after and opposite to the word 'homosexual'.