It's generally defined to be "a musical performance to be played in a local space, in silence outside of headphones etc. without bothering others, through which both of performers and audience (who want to listen) listen to the performed sound.
2012.04 The first theory of silent street live music without bothering others: Three practices - non-noisy, non-occupying, being acknowledged - make a flow field of musical improvisation. http://blog.livedoor.jp/aratori/archives/2017-09-08.html