Definition 1

Stable state associated with little change in beliefs, behavior, commitments, relationships or decisions.

Source: Behavioral Science Lab, 2017

Definition 2

In behavioral economics, inertia is the endurance of a stable state associated with inaction and the concept of status quo bias (Madrian & Shea 2001). Behavioral nudges can either work with people’s decision inertia (e.g. by setting defaults) or against it (e.g. by giving warnings)(Jung, 2019).
In social psychology, the term inertia is sometimes also used in relation to a persistence in (or commitments to) attitudes and relationships.

Source: Behavioral Economics