activation energy

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun chemistry, physics The energy required to initiate a reaction. For example, the flame from the fuse of a firecracker provides a small initial amount of energy, after which the explosive reaction proceeds by itself, releasing a considerably larger quantity of energy. A small push given to a stable but top-heavy object may cause it to fall over; the potential energy released during the fall was present in the system all along but could not be realized as long as the object was upright and balanced.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun the energy that an atomic system must acquire before a process (such as an emission or reaction) can occur