"Habitability" is one of the most misused words in astrobiology. It is a usually a blurry concept used to justify a mission: “The [insert mission name here] will investigate the habitability of [insert world name here]”.
More recently, habitability is becoming an actual quantitative and comparative concept. Specifically, how many microbes can exist in an environment?
Here is an open-access article on Planetary Habitability:
Mendez et al, Astrobiology 2021: https://www.liebertpub.com/doi/10.1089/ast.2020.2342