noun
rubber balloon
A balloon made of rubber, as opposed to paper or other materials. Often used for party balloons or children's toys.
子供たちはゴム風船をふくらませて遊んでいる。
The children are playing, blowing up rubber balloons.
風船 is a general term for any balloon, while ゴム風船 specifically refers to a rubber balloon.
Compound of ゴム (rubber, from Dutch 'gom') and 風船 (balloon).