“The opposite of staccatto may be shown by the word tenuto (often abbreviated to ten.). Any notes to which this direction applies are to be held for their full time value and not shortened in any way. (In some circumstances, tenuto may even imply a slight lengthening, occasionally even a short pause; these however are not the normal implications of the term).”
(Eric Taylor – The AB guide to music theory – Part I)
It seems to me that tenuto applies to a single note, while legato would apply for a sequence of notes (e.g., a phrase)