Have you ever flipped a coin to make a decision or rolled the dice to pursue a path in Dungeons and Dragons? If so, then you know what aleatory is : random chance.
This element of chance has been introduced to music. The composer relinquishes control over certain elements of a composition and allows them to be determined by chance. This could involve things like tempo, dynamics or often the arrangement of the order of the sections of the piece.
This is not really such a new idea in music. Back in the early days of instrumental music, the composer would rarely specify which instrument would play which part. The determination was made by what instruments were available at a particular performance - who showed up and with what instrument i.e. random chance.
Even Mozart wrote some aleatory music! He numbered sections and the order of playing was decided by a roll of the dice.
The introduction of chance means that no two performances are the same; not just a variance of interpretation, it can mean a very different composition each time. In modern compositions this is more striking because so many elements can be given over to random chance.
At the Made in Canada concert September 27th, you will be able to experience aleatory music when "Shadow Prism" by Jason Nobel is performed.
Here is one version.