Full Idea
Certainty might be knowledge of a truth such that to doubt it in a practical way would be insane; and sometimes it is taken more broadly, to cover cases where doubt would be very blameworthy.
Gist of Idea
Certainty is where practical doubt is insane, or at least blameworthy
Source
Gottfried Leibniz (New Essays on Human Understanding [1704], 4.11)
Book Reference
Leibniz,Gottfried: 'New Essays on Human Understanding', ed/tr. Remnant/Bennett [CUP 1996], p.445
A Reaction
The normative aspect of the second half of this touches on a trend in recent epistemology. You have rights to believe, and duties to believe, and virtues for the justifying process. I prefer more neutral, value-free epistemology.