individual item (i.e. not part of a set); single article
single item out of a set; one item from a set
ハンバーガーは単品で。
I'll have the hamburger as an individual item.
単品でのご注文も可能です。
Ordering individual items is also possible.
服はセットではなく単品で販売しています。
Clothes are sold individually, not as sets.