A kitchen utensil for straining liquids or sifting dry ingredients. The kanji spellings 漉し器 and 濾し器 are less common.
When making miso soup, I strain the broth with a strainer.
Sift the flour through a sieve before using it.
Common kanji spelling using こし (straining) and 器 (utensil).
Uses the kanji 漉 (to strain, filter). Less common but still seen.
Uses the kanji 濾 (to filter). Rare and somewhat technical.
Compound of こし (連用形 of こす 'to strain, filter') + 器 (utensil, container).