Merriam-Webster's Collegiate® Dictionary, 11th Edition
ONLY
I. adjective see: one Date: before 12th century unquestionably the best ; peerless ,
2. alone in a class or category ; sole , having no brother or sister , few ,
II. adverb
Date: 14th century
1. as a single fact or instance and nothing more or different ; merely , solely , exclusively , at the very least ,
3. in the final outcome , with nevertheless the final result ,
4. as recently as ; not before , in the immediate past ,
Usage: The placement of only in a sentence has been a source of studious commentary since the 18th century, most of it intended to prove by force of argument that prevailing standard usage is wrong. After 200 years of preachment the following observations may be made: the position of only in standard spoken English is not fixed, since ambiguity is avoided through sentence stress; in casual prose that keeps close to the rhythms of speech only is often placed where it would be in speech; and in edited and more formal prose only tends to be placed immediately before the word or words it modifies.
III. conjunction
Date: 14th century
1. with the restriction that ; but , and yet ; however , were it not that ; except