Historically, a disturbance in an ecosystem is defined as "a discrete event in time that disrupts ecosystems or communities, changes substrate and/or resource availability, and creates opportunities for new individuals or colonies to become established."
Disturbances might be caused by abiotic events like fires, storms, floods, droughts, or changes in water currents. They might have a biological origin, such as trampling by large grazing herbivores, underground tunneling by moles or prairie dogs, bioturbation of marine sediments, deforestation, or off-road vehicles.
Disturbances come in two scales: disasters and catastrophes. Disasters are not so severe as to wipe out everything in the ecosystem, and they tend to happen often enough to exert some natural selection. Depending on the scale of the ecosystem, they can be as small as turning over a rock. Catastrophes have no possible adaptive responses (volcanic eruptions, asteroid impacts, strip mining, etc.) - the slate is wiped clean and any new ecosystems must start from scratch.