In terms, spontaneous (TR:5) (LH:7) (TL:12), from Latin sponte, meaning: "add", means proceeding from natural feeling or native tendency without external constraint; developing or occurring without apparent external influence, force, cause, or treatment; refers to 
The following are quotes:
- “When the only external forces to which the body is subjected consist of a uniform and constant normal pressure, P, on the surface, dW = – PdV, so that if we write G for H + PV, or U – TS + PV, the preceding result becomes: dG = 0. If, under these conditions, V be also constant, then: dH = 0. Let us now suppose that the system is in equilibrium in any state A at any uniform temperature T. Then the equilibrium in the state A will be unstable if a slight shake or touch, by which no perceptible change is made in the system, causes the equilibrium to be broken in consequence of which the system rushes into some other state P. Hence clearly the equilibrium in the state A will be stable if every spontaneous change of state, like AP, is impossible.”
- — John Parker (1891), Elementary Thermodynamics (pg. 328) 
- Spontaneity criterion
- Spontaneous – Hmolpedia 2020.