Multiple realities(covers information from several alternate timelines)
A backpack was a specific type of bag carried on the back.
Cold weather gear often included a backpack. (ENT: "The Seventh", "Regeneration", DS9: "The Ascent", VOY: "Timeless") For the recreational activity of orbital skydiving, a backpack was also used to store the parachute. (VOY: "Extreme Risk", Star Trek)
The orbital skydiving backpacks seen in
Star Trek were created and provided by the company Quantum Creation FX. 
A member of the assault team from Enterprise NX-01 which beamed down to the Vulcan monastery at P'Jem was carrying a backpack. It included phase-pistols, a Vulcan scanner, and a set of micro-charges and a remote control. (ENT: "The Andorian Incident")
The call sheet for this episode is only listing a "small equipment bag" in the props section.
The MACO environmental suits had their support systems located in a backpack. (ENT: "Anomaly")
Chief Petty Officer Garison carried a large radio backpack while being part of the landing party which beamed down to Talos IV in 2254. (TOS: "The Cage")
The water mask life support device made use of a backpack. (TAS: "The Time Trap", "The Ambergris Element")
In 2270, an Orion captain was carrying explosives in a backpack he attempts to use for killing James T. Kirk and destroying the USS Enterprise. (TAS: "The Pirates of Orion")
Strike team uniforms were also equipped with backpacks. (Star Trek V: The Final Frontier)
As a child, Kathryn Janeway recalled going on backpacking trips with her family. Her parents believed it would help her and her sister keep a connection to their pioneer roots. (VOY: "Resolutions")
In chapter five of the Photons Be Free holonovel by The Doctor, his protagonist had to wear a fifty kilogram backpack version of the mobile emitter. (VOY: "Author, Author")
The company The Coop released several Star Trek themed packs and backpacks.
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