A continuous variable, especially one with respect to time in Newton's Method of Fluxions.
"In his 1736 treatise on calculus, Leibniz described the rate of change as a fluent quantity that flows continuously through time within Newton's framework of fluxions."
expressing yourself readily, clearly, effectively
"able to dazzle with his facile tongue"
"silver speech"
That flows; flowing, liquid.
"The chef poured the melted chocolate so fluently that it cascaded in a single, unbroken ribbon over the cake batter."