Definitions for yotta-
Random House Webster's College Dictionary
a combining form used in the names of units of measure equal to one septillion of a given base unit:
Origin of yotta-:
alter. of Gk iôta, ninth letter of the Greek alphabet (y is the second-to-last letter of the Latin alphabet)
In the International System of Units and other metric systems of units, multiplying the unit to which it is attached by 10. Symbol: Y
Origin: ὀκτώ, for the eighth order of 10. The consonants ct are reduced to tt as in Italian, by analogy with peta-; the final a conforms to the finals vowel of the SI series from mega- upwards; the letter y was added, as the second term in a series running through the alphabet backwards (from zetta-).