Singly means one at a time, separately, or individually rather than together or in groups. When students line up singly to get their lunch, they go through one person at a time instead of crowding together. When a librarian asks you to return books singly to the cart, she wants you to place them one by one, not in a stack.

The word often emphasizes the separateness or isolation of something. A scientist might test chemicals singly before mixing them, to see what each one does on its own. A coach might work with players singly during practice, giving each athlete individual attention instead of teaching only the whole team together.

You might also see singly contrasted with “in pairs” or “in groups.” If a teacher says students can work singly or in pairs on a project, you can choose to work alone or with a partner. The word captures that sense of one-by-one-ness, whether you're entering a room, solving problems, or examining ideas.