noun

the quality of behaving or speaking in such a way as to avoid causing offence or revealing confidential information.

she knew she could rely on his discretion

circumspection,care,carefulness,caution,wariness,chariness,guardedness,tact,tactfulness,diplomacy,delicacy,sensitivity,subtlety,consideration,prudence,judiciousness,judgement,discrimination,sense,good sense,common sense,kid gloves