To intervene into the fabric or experience of a given environment.
It can be done by human activity, by the addition of objects / sculptures / light / text, through participation (again on varying scales), or by taking something(s) away.
An intervention can be short or long term, funny, serious, angry, caring, useful, anarchic, antagonistic, cryptic and complex, or simple and direct. The line between art and activism is often blurred in this field; an intervention can spring from motivations that are anything but artistic.