A very small window or window-like opening; a tiny fenestra or aperture.
From Latin 'fenestra' (window) with the diminutive suffix '-ule' meaning very small; used in technical and anatomical descriptions of minute openings.
In bryozoan colonies, fenestrules are the tiny spaces between branches that look like windows under a microscope—it's one of the most beautifully specific scientific words you can use!
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