In lang=en terms the difference between legato and tremolo

is that legato is a slur curve above or below a passage of notes indicating that they should be played in a legato manner while tremolo is a rapid repetition of the same note, or an alternation between two or more notes. It can also be intended to mean a rapid and repetitive variation in pitch for the duration of a note. It is notated by a strong diagonal bar across the note stem, or a detached bar for a set of notes (or stemless notes).

As an adverb legato

is smoothly, in a connected manner.