adverb
(not) a single one; (not) any; (not) at all
Used with a negative predicate to emphasize complete absence or zero quantity. Stronger than just using a negative alone.
冷蔵庫に野菜が一つもない。
There isn't a single vegetable in the fridge.
彼の言うことは一つも信じられない。
I can't believe a single thing he says.