Link disavowal
Disavowal is a file submitted to Google listing inbound links that a site disclaims. The engine then ignores them in its evaluation. The tool is reserved for specific, well-documented situations and should not be used preventively.
In practice
Real use cases: a manual action for artificial links, or the legacy of a large-scale spam campaign. Outside those situations, preventive disavowal does more harm than good — links that were contributing value sometimes get removed. A thorough analysis classifying suspicious domains by type and origin should precede any submission.