The meaning of the English word "Mercy" can really be put into two categories:
1. Compassionate or kindly forbearance shown towards an offender, an enemy, or other person in one's power; compassion, pity, or benevolence. "Have mercy on me a sinner."
2. Something that gives evidence of divine favour, blessing. "It was just a mercy we had our seat belts on when it happened."
Are either of these definitions close to the meaning of the Gurbani words
Karam, Kirpa, Mihar, Daya, Gur Prsaad, etc?
Is there a difference in Gurbani between Nadar (grace) and mercy?
From what I've learned, the only time when the Akaal Purakh actually interferes in the world is to apply Nadar to the lucky Gurmukhs. If that's the case, then all the above words are synonyms.