A flashlight (sometimes called a light) was a portable hand-held light source that can illuminate darkened areas.
In the year 2152 Alana Hart was inspecting the starboard launch tube with a flashlight. (ENT novel: What Price Honor?) Later in that same year, Hoshi Sato used a flashlight while crawling through the air vents on the Enterprise (NX-01). (ENT novel: Shockwave)
When the power was cut on the USS Enterprise (NCC-1701) in 2270, various bridge members used emergency-kit flashlights in an attempt to make repairs. (TOS novel: Crossroad)
Starfleet Intelligence agents Bridget McLellan and Cervantes Quinn used flashlights to explore the caves in the Jinoteur system. (VAN - Declassified novella: The Stars Look Down)
Rom and Etana Kol used flashlights while trying to find the missing bridesmaids on Deep Space 9. (DS9 comic: "Four Funerals and a Wedding")