Partners in committed relationships are also sometimes described as a "significant other" or simply "partner", especially if the individuals are cohabiting.

"Girlfriend" and "partner" mean different things to different people; the distinctions between the terms are subjective. How the term is used will ultimately be determined by personal preference.

In 2005, a study was conducted of 115 people ages 21 to 35 who were either living with or had lived with a romantic partner. It notes that the lack of proper terms often leads to awkward situations, such as someone becoming upset over not being introduced in social situations to avoid the question.

There exists some ambiguity between the terms "girl friend," or a friend who is a girl, and "girlfriend." The transition between the two is a significant aspect of adolescent development.

Both forms of "girlfriend" and "girl friend" are used by different people to mean different things. For example, when the term "girlfriend" is used by a female about another female in a non-sexual, non-romantic context, the two-word form "girl friend" is sometimes used to avoid confusion with the sexual or romantic meaning; however, this is not a rule. In this sense of its usage, "girlfriend" is used in terms of very close friends and has no sexual connotations, unless it is in the case of lesbian, bisexual, pansexual, transgender women. The term "girlfriend" is also used in LGBT communities and can refer to people of any sex or sexuality.

The term "girlfriend" does not necessarily imply a sexual relationship, but is often used to refer to a girl or woman who is dating a person she is not engaged to without indicating whether she is having sex with them. With differing expectations of sexual mores, the term "dating" can imply romantic activity whereas simply using "friend" would likely avoid implying such intimacy. It is essentially equivalent to the term "sweetheart", which has also been used as a term of endearment. A similar relationship wherein there is no exclusivity is sometimes referred to by terms such as friend with benefits.

The word "girlfriend" was first used in 1863 as "a woman's female friend in youth.” In 1922, the word girlfriend was used to mean a man’s sweetheart.

The word "dating" entered the American language during the Roaring Twenties. Prior to that, courtship was a matter of family and community interest. Starting around the time of the Civil War, courtship became a private matter for couples.^[13]

• A female engaged in an extramarital relationship with a married man is frequently considered a "mistress". The word "mistress" was originally used as a neutral counterpart for the words "mister" or "master".^[14]
• The word "madam" is still a respectful form of address, but has had sexual connotations since the early 18th century and has been used to refer to the owner of a brothel since the early 20th century.^[14]
• Some terms of endearment directed to females, a romantic relationship not required, are "darling", "sweetheart", "love", etc.^[15]
• Users of Internet slang and SMS slang often shorten "girlfriend" to the initialism "gf".^[16]
• Additionally, gender-indiscriminate terms also apply (e.g., lover, heartthrob, paramour, squeeze, sweetheart, true love, wooer, date, escort, steady, admirer, bae, or companion).

A similar, but not equivalent, concept is the more ambiguous "lady friend" – a companion of the female gender who is possibly less than a girlfriend but potentially more than a friend. That is to say, the relationship is not necessarily platonic, nor is it necessarily an exclusive, serious, committed, or long-term relationship. The term avoids the overt sexual implications that come with referring to a woman as someone's "mistress" or "lover". In that sense, it can often be a euphemism. The term can also sometimes be employed when someone simply does not know the exact status of a woman that a man has been associating with. For instance, tabloid headlines often note that a celebrity has been seen with a new "lady friend".^[17][18] "Lady friend" may also be used to signify a romantic relationship with an older woman, when the term "girl" as in "girlfriend" may be deemed age-inappropriate.

As of 2007, The New York Times style guide discouraged the use of the term "girlfriend" for an adult romantic partner, stating, "Companion is a suitable term for an unmarried partner of the same or the opposite sex."^[19] The Times received some criticism^[19] for referring to Shaha Riza as the "girlfriend" of World Bank president Paul Wolfowitz in one article about the controversy over their relationship. Other news articles in the Times had generally referred to her as Wolfowitz's "companion".

However, the 2015 edition of the New York Times Manual of Style states that the view on the term "girlfriend" as being informal is now relegated to the realm of traditionalism, and that it has become accepted to use "girlfriend" and "boyfriend" to describe people of all ages (with consideration given to the preferences of the people involved).^[20]

• Domestic partnership
• Girlfriend experience