Improvisation is a word often associated with Jazz.How do people feel about its use in traditonal[English ,Scottish, Welsh, Irish]music?

By improvisation I mean both chordal and melodic.
Are the melodic variations found in the Northumbrian pipers tune book, improvisation?
Finally when improvisation is written down, in the form of sheet music,eg Northumbrian pipers tune book[Felton Lonnen with variations]is it still improvisation,or does it, when written, cease to be improvisation? Dick Miles