noun
one (thing); one piece; one item
Counts one discrete object; used with the counter 個 for small, often three-dimensional items.
りんごを一個ください。
Please give me one apple.
この箱には一個ずつ入れます。
Put them in this box one by one.
Compound of 一 (ichi, 'one') + 個 (ko, counter for small objects). The reading いっこ results from a regular sound change where ち (chi) before a k-row consonant becomes っ (small tsu).