From the American Heritage Dictionary:
ADJECTIVE:1. Alone in kind or class; sole: an only child; the only one left. 2. Standing alone by reason of superiority or excellence.
ADVERB:1. Without anyone or anything else; alone: room for only one passenger. 2a. At the very least: If you would only come home. The story was only too true. b. And nothing else or more: I only work here. 3. Exclusively; solely: facts known only to us. 4a. In the last analysis or final outcome: actions that will only make things worse. b. With the final result; nevertheless: received a raise only to be laid off. 5a. As recently as: called me only last month. b. In the immediate past: only just saw them.
CONJUNCTION:1. Were it not that; except. 2a. With the restriction that; but: You may go, only be careful. b. However; and yet: The merchandise is well made, only we can't use it.
ETYMOLOGY:Middle English, from Old English nlc : n, one; see one + -lc, having the form of; see –ly1.
USAGE NOTE: When used as an adverb, only should be placed with care to avoid ambiguity. Generally this means having only adjoin the word or words that it limits. Variation in the placement of only can change the meaning of the sentence, as the following examples show: Dictators respect only force; they are not moved by words. Dictators only respect force; they do not worship it. She picked up the receiver only when he entered, not before. She only picked up the receiver when he entered; she didn't dial the number. Though strict grammarians insist that the rule for placement of only should always be followed, there are occasions when placement of only earlier in the sentence seems much more natural, and if the context is sufficiently clear, there is no chance of being misunderstood. In the following example only is placed according to the rule: The committee can make its decision by Friday of next week only if it receives a copy of the latest report. Placement of only earlier in the sentence, immediately after can, would warn the reader that a condition on the statement follows.