1) The belief and expectation that someone else will take a necessary action and so freeing you from the obligation.
2) A phenomenon where a necessary action is neglected because all those who can take the action expect others to do so.
3) One cause of passivity and inaction.
Example: An individual waits for a scientist to discover a new technology to address CO2 levels in the atmosphere while continuing to live a carbon-intensive lifestyle and continuing to support fossil-fuel dependency. Diffusion of responsibility is one of the contributing factors to passivity in the face of an imminent danger.