mer·cy 
(mûrsē)
(mûrsē)n. pl. mer·cies
Idiom: 1. Compassionate treatment, especially of those under one's power; clemency.
2. A disposition to be kind and forgiving: a heart full of mercy.
3. Something for which to be thankful; a blessing: It was a mercy that no one was hurt.
4. Alleviation of distress; relief: Taking in the refugees was an act of mercy.
at the mercy of
Without any protection against; helpless before: drifting in an open boat, at the mercy of the elements.
[Middle English merci, from Old French, from Medieval Latin mercēs, mercēd-, from Latin, reward.]
Synonyms: mercy, leniency, clemency, charity
These nouns mean humane and kind, sympathetic, or forgiving treatment of or disposition toward others. Mercy is compassionate forbearance: "The challenge ... is how to define morally reasonable grounds on which to grant perpetrators mercy and allow them to go free" (Pumla Gobodo-Madikizela).
Leniency implies mildness, gentleness, and often a tendency to reduce punishment: "Even though Grant advocated leniency toward the Confederacy's military leaders, he called for punishment of its political leaders" (Brooks D. Simpson).
Clemency is mercy shown by someone with judicial authority: The judge believed in clemency for youthful offenders. Charity is goodwill and benevolence in judging others: "With malice toward none, with charity for all ... let us strive on to finish the work we are in" (Abraham Lincoln).
These nouns mean humane and kind, sympathetic, or forgiving treatment of or disposition toward others. Mercy is compassionate forbearance: "The challenge ... is how to define morally reasonable grounds on which to grant perpetrators mercy and allow them to go free" (Pumla Gobodo-Madikizela).
Leniency implies mildness, gentleness, and often a tendency to reduce punishment: "Even though Grant advocated leniency toward the Confederacy's military leaders, he called for punishment of its political leaders" (Brooks D. Simpson).
Clemency is mercy shown by someone with judicial authority: The judge believed in clemency for youthful offenders. Charity is goodwill and benevolence in judging others: "With malice toward none, with charity for all ... let us strive on to finish the work we are in" (Abraham Lincoln).