from The American Heritage® Dictionary of the English Language, 4th Edition
- n. A useful principle having wide application but not intended to be strictly accurate or reliable in every situation.
from Wiktionary, Creative Commons Attribution/Share-Alike License
- n. A general guideline, rather than a strict rule; an approximate measure or means of reckoning based on experience or common knowledge.
- n. Approximated, guesstimated.
from the GNU version of the Collaborative International Dictionary of English
- n. any rude process or operation, like that of using the thumb as a rule in measuring; hence, judgment and practical experience as distinguished from scientific knowledge.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
- n. a rule or principle that provides guidance to appropriate behavior
Using the rough rule of thumb that one gets about a tenth of whatever you put into the boiler out as distillate, 3,000 gallons means this operation was producing 300 gallons of alcohol at a time, which would add up to 600 gallons of 100-proof white lightning.