A city block was an area surrounded by streets on all sides.
In 2152, Hoshi Sato narrowed down the location of Malcolm Reed's missing communicator on a pre-warp planet to three city blocks. (ENT: "The Communicator")
In 2268, Bela Okmyx suggested he and James T. Kirk meet at an intersection at the end of the block, near a yellow fireplug. Kirk agreed after Montgomery Scott located the intersection. (TOS: "A Piece of the Action")
From the window of Dixon Hill's office, Jean-Luc Picard could see an entire city block, something he marveled at in 2364 when appreciating the holoprogram's realism. (TNG: "The Big Goodbye")