from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. Amount of energy.
From the need of the boiler of a steam locomotive to exceed a minimum amount of pressure (See head ("buildup of fluid pressure"), also pressure head.) before the locomotive could start moving. (Wiktionary)
The Migs were not too pleased when they found that by then we were really carrying a head of steam and that their aircraft could not compete with us down low.