to make (something) less harsh or hostile
to make (something) less severe or painful
to lessen the seriousness of (something)
mitigating circumstances
mitigable adj
mitigation /-ʹgaysh(ə)n/ noun
mitigator noun
mitigatory /-gaytəri, -gət(ə)ri/ adj
[Middle English mitigaten from Latin mitigatus, past part. of mitigare to soften, from mitis soft]