Commonplace means ordinary, unremarkable, or happening so often that people barely notice anymore. When something becomes commonplace, it's no longer special or surprising because everyone has seen it many times before.

A hundred years ago, owning a car was extraordinary, but today it's commonplace in most American towns. Smartphones seemed like science fiction in the 1990s, but now they're so commonplace that almost every adult carries one. When your teacher assigns homework, that's pretty commonplace, nothing unusual or unexpected.

The word often suggests that something used to be remarkable but lost its novelty. Video calls seemed amazing when they first became possible, but now they're commonplace in schools and offices. Sometimes people use commonplace with a slightly disappointed tone, as when someone says, “Special effects in movies have become so commonplace that nothing impresses audiences anymore.”

Notice the difference between commonplace and common. Rain might be common in Seattle (it happens frequently), but seeing a friend at school is commonplace (it's so ordinary you don't think twice about it). Something commonplace has become part of the everyday background of life, like buses, traffic lights, or pencils: useful and everywhere, but nothing you'd call exciting.