Noun, pl.fontanelles
Also spelled as fontanel.
1. Any membranous gap between the bones of the cranium in an infant or fetus. (wordnetweb.princeton.edu)
2. A soft membraneous spot on the head of a baby due to incomplete fusion of the cranial bones. (wiktionary.org)
3. An anatomical feature on an infant’s skull comprised by any of the soft membranous gaps between the incompletely formed cranial bones of a foetus or an infant. Fontanelles allow for rapid stretching and deformation of the cranium as the brain expands faster than the surrounding bone can grow. (wikipedia.org)
Word origin: From French fontanelle (16c.), from Old French fontenele “small source, fountain, spring; fontanelle,” diminutive of fontaine “spring” (see fountain),