It all depends on the time period.
In modern terminology, it would be a "grace note" (eng) or "acciaccatura" (it).
The problem, though, is that in Classical era music, they are not grace notes. They are appoggiaturas, or a type of non harmonic tone. Since they were "naughty" notes at the time, they were written smaller, but played in rhythmic time with the rhythmic value they were given, albiet written smaller.
With this in mind, editors often put dashes through the small notes, making them written as true grace notes, however in classical music they should never have dashes through them nor be played as grace notes.
I have a whole three chapters on this in a book somewhere, but I can't find it at the moment.
I don't understand the term "light" notes because I have often seen accents on them. Just because they are written smaller doesn't make them light.