In English the function that this would serve is actually served by the noun methyl. RHU, the sole OneLook dictionary that has methyl- calls it a combining form, not a prefix.
Apparently in Italian, at least, there is a prefix distinct from the corresponding noun. But there is no reason why an English prefix entry needs to be here for translations. The gloss of the Italian prefix could be the English noun and both the Italian prefix and the noun could be translations of the English noun. DCDuringTALK 04:32, 28 June 2011 (UTC)
Acturally, no. In IUPAC organic nomenclature "methyl-" functions as a prefix, and not always in a way that would agree with the noun "methyl". Names like 3-methylhexane or 6,6-dimethyl-4-ethylundecane are not using "methyl" as part of a compounded name indicating chemical origin, but as a prefix to denote the existence of a sidechain. The IUPAC rules explicitly call these forms prefixes. --EncycloPetey 03:18, 1 July 2011 (UTC)
Keep per EP. I don't think IUPAC's use of the term "prefix" is binding on us — if they called it a "prefixed", we wouldn't be forced to adopt a ===Prefixed=== header — but it seems reasonable in this case. By the way, although I think the distinction between "combining form" and "prefix" is worth making, combining forms really are prefixes, just as proper nouns really are nouns, and so on. —RuakhTALK 21:31, 1 July 2011 (UTC)