A spontaneous process is "a physical or chemical change that occurs without the addition of energy."^[1] It can also be described as a "process that will occur without any energy input from the surroundings [or a] process that will occur on its own."^[2] The process can be fast or slow; "spontaneous" says nothing about its speed.^[3]

A process that is capable of proceeding in a given direction, as written or described, without needing to be driven by an outside source of energy. The term is used to refer to macro processes in which entropy increases; such as a smell diffusing in a room, ice melting in lukewarm water, salt dissolving in water, and iron rusting.

The laws of thermodynamics govern the direction of a spontaneous process, ensuring that if a sufficiently large number of individual interactions (like atoms colliding) are involved then the direction will always be in the direction of increased entropy (since entropy increase is a statistical phenomenon).

References

1. ^ chem.purdue.edu
2. ^ chemistry.about.com
3. ^ Chapter 10: Spontaneity, Entropy and Free Energy