Project Ara was a modular smartphone scheme handled by Google. For simplicity, the term 'modular' means: having parts that can be connected or combined in different ways.

The idea of a modular smartphone where some of the parts can be replaced was somewhat promising. But as of now, the project has been terminated (since Sep 2)- owing to cost and economic value to start with (on the high side). And it also had some flaws in engineering and more.

Replaceable features include : processors, displays, batteries, and cameras, as well as modules providing more specialized components, and "frames" that these modules were to be attached to.

Google eventually narrowed it down to the camera, the battery and stuff.

My belief is - though Project Ara failed, something like this can still be achievable in the future.

LG already pioneered modular smart phones with it's LG G5

GogetterMD:
LG already pioneered modular smart phones with it's LG G5

That's not modular, it's semi-modular.

Modular phones are still under development. POINT OF CORRECTION : If it's modular, then all of the parts can be replaced. But if it's semi-modular, then some of the parts can be replaced. And if it's non-modular, none of the parts can be replaced.

The Project Ara Smartphone Design (The Verge)