approximant

noun

ap·prox·i·mant ə-ˈpräk-sə-mənt

plural approximants

phonetics : a speech sound that is formed by the passage of air between two articulators (such as the lips or tongue) which are close but not touching and that is usually classified as a consonant

The initial consonant in raw is also an approximant, at least for those speakers of English who do not trill or flap the r.—

John Clark et al., An Introduction to Phonetics and Phonology, 2007

mathematics : a mathematical method or function that is used to find an approximation

While the expression n/log n is a fairly simple approximation for π(n), it is not terribly close, and mathematicians have been interested in improving it. Of course, one does this at the price of complicating the approximant.—

Philip J. Davis, The Mathematical Experience, 1999