I. (ˌ)in-ˌfi-nə-ˈte-sə-məl, -zə-məl noun
Etymology: New Latin infinitesimus infinite in rank, from Latin infinitus
Date: 1706
: an infinitesimal quantity or variable
II. adjective
Date: 1710
1. : taking on values arbitrarily close to but greater than zero
2. : immeasurably or incalculably small
an infinitesimal difference
• in·fin·i·tes·i·mal·ly -mə-lē adverb