The thing is, there isn’t really a set definition—it’s more of a colloquial distinction, and there are some conflicting opinions. “Fine lines are often considered to be baby wrinkles,” says board-certified dermatologist Joshua Zeichner, M.D., as the faint etchings can mature into deeper folds later on. “Others consider them to be separate entities, not related to facial expression but more a function of UV light,” he adds, since UV rays alter the underlying elastic structure in the skin, which can cause fine, tissue-like creases (aka crepey skin). 

But again, there’s no hard rule to separate the two in dermatology. “If you get five dermatologists in a room, you’ll likely get five different opinions on what constitutes a wrinkle versus a line,” adds Zeichner. Of course, you can always assess the crease itself: A fine line will often appear, well, fine and faint, while a wrinkle may be more static and deep-set. But even this can be subjective, depending on what you consider “fine.”