Webster's English Dictionary

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infinitesimal

1. in.fin.i.tes.i.mal \(.)in-.fin-*-'tes-*-m*l, -'tez-\ n [NL infinitesimus 
   infinite in rank, fr. L infinitus] 1: a function that can be made 
   arbitrarily close to zero  2: an infinitesimal quantity 
2. infinitesimal \-m*-le-\ aj 1: capable of being made arbitrarily close to 
   zero  2: immeasurably or incalculably small  - in.fin.i.tes.i.mal.ly av