Is “girl” a valid synonym for “young woman”?

Solution 1:

First, a necessary disclaimer: context and audience matter a lot in what is considered insulting. For instance, two people may interpret being called girl differently; calling a coworker or boss a girl has different connotations compared to calling your best friend a girl. This answer will not attempt to parse all of the nuances of context. Rather, it will give a general answer that (a) girl is sometimes used to refer to young women and (b) it is sometimes classified as offensive.

Young Women / Offensive?

I start with the Oxford Learner's Dictionary (OLD), because it poses the issue in brief:

​ 3. [countable] (sometimes offensive) a young woman

(example) Alex is not interested in girls yet.

(example) He married the girl next door.

Sometimes offensive, in a learner's dictionary, is a warning to any language learner: be careful when you use this term. Yet the examples seem harmless in themselves. The girl next door, in particular, is practically an archetype or cliché with its own Wikipedia page. So what makes this meaning of girl potentially offensive?

It comes down to register. First, girl also refers to children, so calling even a young woman a girl may convey connotations of immaturity. This is a line of thought that hearkens back several decades. In short, feminists in the 1960s and 1970s urged people to consider the power imbalances in language choices, which included the colloquial use of girl to refer to women in professional situations. By this decade, that expectation has virtually become a social norm, so that someone as standard as Ask A Manager's Alison Green answers a query about using "girl" in the workplace with this:

While referring to adult women as “girls” may not be intended to be infantilizing or patronizing, language has power, and girls are rarely taken as seriously as women. Some of the most damaging sexism is subtle because it impacts how we think without us even realizing it.

Second, she also points out a double-standard:

And for anyone reading who thinks that referring to women as “girls’ in a professional context is no big deal, consider how infrequently you hear “we have a boy who codes our emails.”

In the Harvard magazine Fifteen Minutes, anthropology professor Susan Greenhalgh explains that double standard further, as there is no term like guys to serve as a middle-ground in register for girls/women:

"The terms 'male' and 'female' are gender-neutral, but not especially desirable for describing social issues, since they signal biological aspects of gender (...) For college-aged 'males,' we have the helpful term 'guys,' which allows us to avoid both 'men' and 'boys.' For 'females,' there is no similar term (the comparable term, 'gals,' having gone out of fashion a long time ago), forcing us to choose between 'girls' and 'women.'"

So girls has to do more work, and accordingly the term introduces more conflict, especially for young women who have an understandable desire to be taken seriously compared to their peers and colleagues. Issues of register thus make using girl a risk, especially in more formal or unfamiliar contexts.

When Is It Okay?

First, set phrases like girl next door tend to be okay, albeit in more informal contexts.

Second, even dictionaries document several acceptable uses for girls. They usually involve an interpersonal relationship. Here is the OLD again:

2. [countable] a daughter

5. [countable] (old-fashioned) a man’s girlfriend

6. girls [plural] (used especially as a form of address by women) a woman’s female friends

2 and 6 seem obvious. A parent has very good reasons to see their child as their girl or boy. Friends are on an equal level, so nights with "the girls" (like nights with "the boys") seem common, and "hey girl" as a form of address from one woman to another fits that pattern. 5, too, is understandable – what one calls one's own partner is often up to those two, and a very personal form of address can be appropriate. So familiarity makes using girl more likely.

Otherwise, context and audience take hold. If the term could be understood as referring to someone's maturity level, it is an insult. Similarly, if the person has some degree of status, it can be an insult. There may be other risks, including an audience's personal experiences with the term and their current status. Because communication is affected by both context and audience, my advice is to understand them both as well as is feasible before saying something that may insult others.