Activation Energy
Etymology
Coined by chemist Svante Arrhenius in 1889 for the energy barrier a reaction must clear. "Activation" traces to Latin "activus," from "agere" ("to do, drive"); "energy" comes from Greek "energeia" ("activity"). The chemical sense migrated into everyday talk about the cost of merely getting started.
noun phrase · /ak-tih-VAY-shun EN-er-jee/
The minimum jolt of energy needed to start a reaction that, once going, may release far more than it took to begin.
What it names
Reach for it to explain why hard-to-start things — a habit, a project, a cold engine — need a disproportionate initial push before they run on their own.
Example
The gym membership was cheap; the activation energy of actually walking in the door was the real price.
Related idea
Distinct from inertia, which is the tendency of a state to persist — activation energy is specifically the hump you must climb to change that state, not the resistance itself.