spontaneity
Spontaneity is just the everyday idea that some things happen by themselves and some don't. A dropped glass shatters; the shards never leap back together. Heat flows from your warm hand into a cold spoon, never the reverse. Iron left in damp air rusts on its own. A spontaneous change is one nature is willing to make without being continuously forced — even if it needs a little nudge to get started.
In thermodynamics the test is sharp. A change is spontaneous when it increases the total entropy of the universe (the system plus its surroundings) — that is the second law. For the common case of constant temperature and pressure, this is repackaged into the tidy rule that the system's Gibbs free energy must fall: spontaneous when ΔG is negative, at a standstill (equilibrium) when ΔG is zero, and forbidden on its own when ΔG is positive.
Two honest cautions. First, spontaneous says nothing about speed — a spontaneous reaction can take a microsecond or a million years, since that is the separate question of kinetics. Second, 'spontaneous' does not mean 'releases heat': plenty of self-starting changes, like an ice pack going cold or a salt dissolving, actually soak heat up, driven instead by a big rise in entropy.
An instant cold pack is spontaneous yet endothermic: tear the inner pouch, the salt dissolves on its own, and the pack turns icy — because the jump in entropy as the crystal spreads into solution outweighs the heat it pulls in.
Spontaneous can still mean cold: entropy, not heat, has the final say.
Beware the word's everyday flavor: 'spontaneous' in chemistry means thermodynamically favorable, not 'sudden' or 'instantaneous.' Whether it actually proceeds at a noticeable rate is governed by activation energy and the reaction path, which thermodynamics ignores entirely.