1. The simultaneous decline or worsening of conditions, relationships, or qualities affecting two or more parties equally
The mutual deterioration of their relationship became evident after months of constant arguments.
A deterioração mútua de seu relacionamento tornou-se evidente após meses de discussões constantes.
2. In legal or contractual contexts, the reciprocal decline in performance or condition of obligations
Without intervention, there will be mutual deterioration of both parties' interests.
Sem intervenção, haverá deterioração mútua dos interesses de ambas as partes.