1.
only child
A child who has no siblings.
私は一人っ子です。
I'm an only child.
一人っ子だから兄弟がほしかった。
Since I'm an only child, I wanted siblings.
最近は一人っ子の家庭が増えている。
Families with only children are increasing recently.
SOCIAL CONTEXT:
With declining birth rates in Japan, 一人っ子 families have become increasingly common.
STEREOTYPE:
There's a common stereotype that only children are spoiled (甘やかされている}) or selfish, though this is often not accurate.
VARIANT:
一人子 is a more formal alternative.