From Envirowiki

Environmental impact generally refers to the adverse impacts that humans have on the environment.

Strictly speaking though, humans can have a positive environmental impact (ie. compost adding nutrients to soil), and other species can also have adverse impacts (such as invasive species).

Major environmental impacts are usually something to do with pollution (ie. toxics, climate change), physical damage (ie. suburb expansion, deforestation), or resource over-exploitation (over fishing, etc.).