If you’ve been hearing the phrase “gender fluid” lately and aren’t exactly sure what it means, you’re probably not alone. The term is being used more frequently these days as society becomes more inclusive, and more people believe that a person’s gender identity can be something other than the conventional gender archetypes of male and female.

Before getting into the definition of gender fluid, it helps to understand what gender identity means—as well as the words cisgender and transgender. Austin, Texas-based clinical psychologist Jo Eckler, PsyD, tells Health that it’s a person’s deep sense of their gender.

“It may or may not match the sex you were assigned at birth or how other people see you,” she says. “If it matches, then you fall into the cisgender category. If it doesn’t match, then transgender or gender non-conforming might be a better fit.” Basically, gender identity is your own internal experience regardless of what your chromosomes, organs, or external physical characteristics might indicate, she adds.

Gender fluid is one of the terms people may use to describe their gender. It’s not to be confused with “non-binary,” but it does fall under the non-binary blanket. “People whose gender is not male or female use many different terms to describe themselves, with nonbinary being one of the most common,” states the National Center for Transgender Equality on the organization’s website. “Other terms include genderqueer, agender, bigender, and more. None of these terms mean exactly the same thing—but all speak to an experience of gender that is not simply male or female.”

“Gender fluidity is a gender identity that may change over time or according to relational or psychological state, and also incorporates the feeling of not having a gender, Virginia-based psychotherapist April Bennett tells Health. Being gender fluid means something different to each person. “In practice, gender fluid people may express masculinity, femininity, or an androgynous self in their personality, sexual experiences, and self-concept,” says Bennett.

Because gender fluidity means not having a fixed, single sense of your gender, that gender could shift over time—during the course of a day, weeks, months, or years. “Whatever form gender fluidity takes, it is important to remember that it is a valid gender identity. It is not being flaky or ‘going through a phase,’” says Eckler. “So many other aspects of ourselves ebb and flow and shift that it only makes sense that our gender can, too.”

Because gender fluidity typically involves shifts or changes in gender, different pronouns may be used, depending on how the person views their gender at that moment in time. Many—but not all—gender fluid people use “they” instead of “he” or “she.” Others prefer “zie” or “Mx.” If you’re not sure, just ask.

Eckler says it’s not known how many people are gender fluid. “Sometimes statistics on the transgender population include gender fluid people, and sometimes they don’t,” she explains. “Even then, it isn’t clear how many gender fluid people there are. Also, due to the lack of common knowledge about nonbinary and gender fluid identities, there are likely people out there who are gender fluid but don’t realize that there is a name for it.”