Definitions
from The American Heritage® Dictionary of the English Language, 4th Edition
 transitive v. To increase the amount, number, or degree of.
 transitive v. Mathematics To perform multiplication on.
 intransitive v. To grow in amount, number, or degree. See Synonyms at increase.
 intransitive v. To breed or propagate.
 intransitive v. Mathematics To perform multiplication.
 adv. In many or multiple ways.
from Wiktionary, Creative Commons Attribution/ShareAlike License
 adv. In many or multiple ways.
 v. To increase the amount, degree or number of (something).
 v. To perform multiplication on (a number).
 v. To grow in number.
 v. To breed or propagate.
 v. To perform multiplication.
 v. To be a factor in a multiplication with (another factor).
 n. An act or instance of multiplying.
from the GNU version of the Collaborative International Dictionary of English
 transitive v. To increase in number; to make more numerous; to add quantity to.
 transitive v. To add (any given number or quantity) to itself a certain number of times; to find the product of by multiplication. See the Note under Multiplication.
 transitive v. To increase (the amount of gold or silver) by the arts of alchemy.
 intransitive v. To become greater in number; to become numerous.
 intransitive v. To increase in extent and influence; to spread.
 intransitive v. To increase amount of gold or silver by the arts of alchemy.
from The Century Dictionary and Cyclopedia
 To make manifold; increase in number or quantity; make more by natural generation or reproduction, or by accumulation, addition, or repetition: as, to multiply men or horses; to multiply evils.
 In arithmetic, to perform the operation of multiplication upon. See multiplication, 2.
 To increase (the precious metals) by alchemical means. See multiplication, 3.
 To grow or increase in number or extent; extend; spread.
 In arithmetic, to perform the process of multiplication. See multiplication, 2.
 To increase gold or silver by alchemical means.
 In a manifold way.
from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.
 v. combine by multiplication
 v. have young (animals) or reproduce (organisms)
 v. have offspring or produce more individuals of a given animal or plant
 v. combine or increase by multiplication
 adv. in several ways; in a multiple manner
Etymologies
Examples

You want the line art layer to be in multiply mode, this effectively makes the white parts transparent, letting the colour layer underneath show through.
EXTRALIFE – By Scott Johnson  Diary of a Cartoonist: Episode 11

When the "*" token is matched, we call the multiply instruction, and so on.

The highest ranked sequences are then allowed to "multiply"  copied and subjected to random changes.

The cells are no longer able to divide and multiply, which is why retinoblastoma occurs very rarely after the age of 5 years.
Retinoblastoma  Diagnosing and Treating Retinoblastoma in Children

Transliteration from Russian is standardized, but transliteration from Kazakh offers several options Kazakh is written with Cyrillic letters, but at least two additional characters, so the possibilities in English multiply.

They showed that the reticulum can be described as a multiply folded, more or less deflated sack occupying most of the cytoplasm.

A pasture can be improved by encouraging the good grasses to grow and multiply, that is, grasses which are richest in fodder units.

Many of the issues involved are of a so highly technical nature as to confuse the layman, especially when Hebrew terms multiply, that he believes the issues must be left to professional theologians and is all too ready to follow their guidance if they adopt, as is often the case, a tone of utter finality.

The bugle calls multiply till the woods seem filled with an advancing army and the yells split the sky.

You might want to practice this now, in your mind (or in the mirror!), before summer hits and catcalls multiply exponentially:
wiredweird commented on the word multiply
(MultiPLY) perform an operation in arithemtic
(MULtiPlee) More than once
March 27, 2007