Orchestra Without a Score

There are different potential terms depending on what they are really doing:

• Playing from memory means they are following a composed piece of music, but they have memorized it.
• Playing by ear means playing music following what they hear rather than what they see (written music).

Improvising, or jamming means they are improvising, but does not imply whether or not they are reading music.

When referring to a style or the name of a group, the word "free" can imply not using written music. For example, "free jazz". I suppose if someone referred to their musical style as "free orchestral music", it would be understood that it's completely improvisational.

I'd call it "extemporaneous music". I do not know if this is an accepted term, but I think it would be understood to mean music without a score (regardless of how many people are playing it). There does seem to be a band by that name now, to add to the confusion. This article on Wikipedia (http://en.wikipedia.org/wiki/Jazz_piano) seems to use it the way I did.

"Playing by ear"; it means learning and/or performing without any musical notation. The opposite would be "playing by rote."