A city block, urban block or simply block is a central element of urban planning and urban design. A city block is the smallest area that is surrounded by streets. City blocks are the space for buildings within the street pattern of a city, they form the basic unit of a city's urban fabric. City blocks may be subdivided into any number of smaller land lots usually in private ownership, though in some cases, it may be other forms of tenure. City blocks are usually built-up to varying degrees and thus form the physical containers or 'streetwalls' of public space. Most cities are composed of a greater or lesser variety of sizes and shapes of urban block. For example, many pre-industrial cores of cities in Europe, Asia and the Middle-east tend to have irregularly shaped street patterns and urban blocks, while cities based on grids have much more regular arrangements.

Possibly derived from the Irish Gaelic bealach, pronounced b'alaċ, meaning a road, a path, a way, a passage, a thoroughfare.Daniel Cassidy noted in his work on American-Irish slang, "that up until the mid-19th century most of New York's population lived below Canal Street, where the block was often part of a tangled weave of paths and old Indian trails," rather than a traditional rectangular grid.