Etymology: Middle English thridde, thirde, from Old English thridda, thirdda; akin to Latin tertius third, Greek tritos, treis three — more at three
Date: before 12th century
a. being next after the second in place or time <the third taxi in line> b. ranking next after the second of a grade or degree in authority or precedence <served as third mate> c. being the forward speed or gear next higher than second especially in a motor vehicle 2. a. being one of three equal parts into which something is divisible <a third share of the money> b. being the last in each group of three in a series <take out every third card> • third or thirdly adverb II. noun Date: 14th century 1. one of three equal parts of something <a third of the pie> 2. a. — see number table b. one that is next after second in rank, position, authority, or precedence <the third in line> 3. a. the musical interval embracing three diatonic degrees b. a tone at this interval; specifically mediant c. the harmonic combination of two tones a third apart 4. plural merchandise whose quality falls below the manufacturer's standard for seconds 5. third base 6. the third forward gear or speed especially of a motor vehicle
New Collegiate Dictionary. 2001.