Strictly speaking (and I have this from an Assay Office), silver plate means solid silver (not pure silver, but one of the standard alloys of silver which come in various degrees of fineness, similar to 9 carat, 14 carat and 18 carat gold). This should be carefully distinguished from "silver plated", but, unfortunately, it frequently isn't. here's the quote from the Assay Office (Birmingham):
You are quite right that the term 'plate' is the traditional and correct word for solid precious metal; in the past, used more to refer to silver, but more recently to all precious metals. 'Plated', therefore, refers strictly to articles of base metal constitution with a thin covering of precious metal.