Citing someone directly with additional information in footnote
My preference would be to avoid the footnote altogether, as I find them distracting. Keep the quote verbatim, give the source, and then explain the terminology in the next sentence of your text:


  Smith claims that "the best breakfast is a soft boiled ovum" [Smith87]. (Smith's "ovum" refers to what our paper calls an "egg.")


Another option, if you don't really care about  showing  Smith's exact words to the reader, is to make the change in the quote itself, indicating the change with brackets.


  Smith claims that "the best breakfast is a soft boiled [egg]" [Smith87].