Straight is a slang term for heterosexual, which is the attraction to people of the "opposite" gender, or the attraction to gender(s) unlike one's own. It is considered the opposite of gay and lesbian. More specifically straight is most commonly used to mean men attracted to women and women attracted to men. The term "straight" is used to describe for both straight men and straight women.
An alternate definition of straight is someone who is attracted to those of a different gender, or someone who is attracted to gender(s) unlike one's own. The usage of the word "opposite" in the standard definition implies there that are only two genders, men and women, and that those genders are opposites. A relationship between two people of different gender is often called "straight" however this is not always accurate as one or both people might be multisexual. A relationship with two people of different genders caused by one of the parties being non-binary can be called diamoric. Non-binary people who feel they are straight but in a uniquely non-binary way may identify as strayt.
While straight is often used to describe non-LGBT people it is possible for straight people to be part of the LGBT community. For example, they might be transgender. They also might be, asexual heteroromantic, or aromantic heterosexual, which also makes them LGBT.
Etymology
The prefix hetero- comes from the Greek word heteros, meaning "different". The term "heterosexual" was coined shortly after and as the opposite to "homosexual".