sextillion

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun The cardinal number equal to 10²¹.
• noun Chiefly British The cardinal number equal to 10³⁶.

from The Century Dictionary.

• noun According to English and original Italian numeration, a million raised to the sixth power; a number represented by unity with thirty-six ciphers annexed; according to French numeration, commonly taught in America, a thousand raised to the seventh power; a thousand quintillions.

from the GNU version of the Collaborative International Dictionary of English.

• noun According to the method of numeration (which is followed also in the United States), the number expressed by a unit with twenty-one ciphers annexed. According to the English method, a million raised to the sixth power, or the number expressed by a unit with thirty-six ciphers annexed. See numeration.

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

• noun A trillion billion: 1 followed by 21 zeros, 1021.
• noun A million quintillion: 1 followed by 36 zeros, 1036.

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

• noun the number that is represented as a one followed by 21 zeros