rubber stopper; rubber plug
A stopper or plug made of rubber, used to seal bottles, test tubes, sinks, etc.
試験管にゴム栓をする。
Put a rubber stopper on the test tube.
風呂のゴム栓がなくなった。
The rubber plug for the bath is missing.
All-katakana variant; it is no-kanji, but it is less common than the mixed spelling.
Compound of ゴム (rubber, from Dutch 'gom') and 栓 (stopper, plug).