Pipe
Cylinder, Tube
noun
The kanji and the word are exactly the same. That means they share meanings as well. You'll see this used to mean pipe more often than cylinder, though.
Since this word is made up of a single kanji, it probably uses the kun'yomi reading. You did not learn this reading, so here's a mnemonic to help you to remember: When it comes to pipes, you always want to have two sets of two of them, just in case. So, that's a two two (つつ) set of pipes, lined up next to each other for backup.
使い終わったら、体温計を筒の中に戻しておいてね。
Can you put the thermometer back in its tube when you’re done using it?