Definition and background:
a jazz term which instructs the performer to play the noted pitches as they are printed. Parts for jazz musicians in big bands often consist of lengthy sections of empty bars labelled with the changing time signatures and chord changes. Rhythm section members improvise an accompaniment (see comp), and lead instruments improvise solos. In sections where the jazz arranger wants the performers to read notated pitches rather than improvise, they indicate this with the notation "as is".
For more information on the music term "As is" check out these other resources:
Wikipedia - Glossary of Musical Terminology
Virginia Tech Multimedia Music Dictionary
ORB -- Medieval Music Glossary