prefix

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

• transitive verb To put or attach before or in front of.
• transitive verb To settle or arrange in advance.
• transitive verb To add as a prefix.
• transitive verb To add a prefix to.
• noun Grammar An affix, such as dis- in disbelieve, attached to the front of a word to produce a derivative word or an inflected form.
• noun A title placed before a person's name.

from The Century Dictionary.

• noun A word or syllable, or a number of syllables, rarely more than two, and usually one (sometimes reduced to a single consonant not forming a syllable), affixed to the beginning of a word, to qualify its meaning or direct, its application: opposed to suffix or postfix, a like addition at the end of a word.
• noun The act of prefixing; prefixion.
• To fix or put before; place in front; put at the beginning.
• To fix beforehand; set or appoint in advance; settle beforehand.

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

• noun That which is prefixed; esp., one or more letters or syllables combined or united with the beginning of a word to modify its signification.
• transitive verb To put or fix before, or at the beginning of, another thing.
• transitive verb obsolete To set or appoint beforehand; to settle or establish antecedently.

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

• noun That which is prefixed; especially one or more letters or syllables added to the beginning of a word to modify its meaning; as, pre- in prefix, con- in conjure.

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

• verb attach a prefix to
• noun an affix that is added in front of the word