What does “objective lens” mean in photography?

By definition, an objective lens is a lens which converges light to produce what's called a "real image" — a projection that can be observed on a screen, or (as in the case of camera) recorded on film or a sensor.

Binoculars and microscopes have objective lenses which converge the light and then also eyepiece lenses which diverge it again.

A stack of glass or apertures which does not produce such an image is not an objective lens. In fact, without a curved surface, you don't really have a lens at all, let alone an objective lens (and the term "objective" in this context is really just short for "objective lens").

In conversational English in the context of photography, we generally call the thing that screws to the front of a camera body a "lens", not an "objective". We even do that when the "lens" in question contains mirrors — we say "a mirror lens". Or, even though it's not technically a lens at all, we say "pinhole lens".