noun
1. : a consonant letter occurring twice in succession in a word (as nn in tunnel )
2.
a. : an acoustic impression apprehended or functioning as two consonants, produced by prolonging an articulation (as of s in bus seat ), by repeating an articulation (as of r in Spanish parra, or by prolonging the interval between successive components of an articulation (as between the occlusion and the release of t in coattail )
b. : a consonant produced by a simultaneous double articulation (as a p pronounced with release of both lips and glottis)
c. : two different consonant sounds occurring in succession (as mp in stamp )