1.
only child
A child who has no siblings.
一人っ子 refers to a child with no siblings. The っ子 suffix gives a colloquial, familiar tone.
Common patterns
Cultural note
With Japan's declining birth rate (少子化), 一人っ子 families have become increasingly common. There is a common stereotype that only children are spoiled (甘やかされている) or selfish, though this is often inaccurate.
Forms
一人子 is a more formal/literary alternative.