residue
[C/U] Both
pl: residues
This term carries a strong connotation of leftovers that are often unwanted, annoying, or chemically distinct from the original substance. It suggests a lingering presence, such as the film left by a cleaning agent or the dust remaining after a reaction, evoking a sense of impurity or incompleteness.
Countable when referring to distinct types of leftovers (e.g., different chemical residues found at a crime scene). Uncountable when referring to a general mass of remaining material (e.g., there is some residue on the glass).