noun
speck
SPEK/spek/
A very small particle, spot, or amount.
A tiny speck sits on the otherwise clean plate.

Etymology
Old English specca meant a small spot or stain. The sense broadened from a visible mark to a tiny bit of something; the word's deeper origin is unknown.
In context
presence of a small impurity a small speck of dust or for some reason if there is a small very small level droplet that is sufficient to trigger the process of condensation