The act of ensuring or making certain; a guarantee or assurance.
From 'ensure' (from Old French 'enseurer') plus '-ance', the noun suffix. While 'insurance' became the standard form for the concept of guaranteed protection, 'ensurance' appears in some historical legal texts.
Here's a linguistic fork in the road: 'ensure' gave birth to both 'insurance' (which took over business and law) and 'ensurance' (which basically disappeared), showing how one spelling variant can completely dominate while another becomes archaic.
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