A hammock was a type of bed that was suspended in the air.
Jono wasn't able to sleep in a Human bed and installed a hammock in Captain Jean-Luc Picard's quarters aboard the USS Enterprise-D when he was staying with the captain for a short time in 2367. (TNG: "Suddenly Human")
In 2371, Benjamin and Jake Sisko had hammocks installed on their Bajoran lightship. However, for Jake they took some getting used to. (DS9: "Explorers")
In 2381, the Klingon officers that served on the lower decks of the IKS Che'Ta' slept on hammocks. (LD: "Wej Duj")
While stranded on an abandoned USS Discovery, the Human known as Craft set up a hammock in its transporter room. (ST: "Calypso")