To make less severe, intense, harsh, rigorous, painful, etc.; to soften; to meliorate; to alleviate; to diminish; to lessen; as, to mitigate heat or cold; to mitigate grief.

Reduce severity. Risk mitigation – reducing the severity or probability of significant risk by action such as warning people away, or building structures able to avoid or withstand lahar forces.

To lessen or diminish the effects of an impact.

to make [a problem or condition] less serious or less harmful.