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pico-

  1. a combining form meaning “one trillionth” (10−12): picogram.
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Origin of pico-

< Spanish pico peak, beak, bit. See pic2
Dictionary.com Unabridged Based on the Random House Unabridged Dictionary, © Random House, Inc. 2018
British Dictionary definitions for pico-

pico-

prefix
  1. denoting 10 –12picofarad Symbol: p
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Word Origin

from Spanish pico small quantity, odd number, peak
Collins English Dictionary - Complete & Unabridged 2012 Digital Edition © William Collins Sons & Co. Ltd. 1979, 1986 © HarperCollins Publishers 1998, 2000, 2003, 2005, 2006, 2007, 2009, 2012

Word Origin and History for pico-

word-forming element used in making names for very small unites of measure, 1915 (formally adopted c.1952 as a scientific prefix meaning "one trillionth"), from Spanish pico "a little over, a small balance," literally "sharp point, beak," of Celtic origin (cf. Gaulish beccus "beak").

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Online Etymology Dictionary, © 2010 Douglas Harper
pico- in Medicine

pico-

pref.
  1. One-trillionth (10-12):picogram.
  2. Very small:picornavirus.
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The American Heritage® Stedman's Medical Dictionary Copyright © 2002, 2001, 1995 by Houghton Mifflin Company. Published by Houghton Mifflin Company.