"Chicago" is the name of a font designed for the user interface of the original Macintosh. (All the early Mac fonts were named after cities, such as Geneva, New York, and San Francisco.)

The first several Macintosh models had black-and-white displays. No, not grayscale -- they could only display black and white. Strictly 1-bit color. The UI designers needed a font which would still be readable when dithered, so that they could display "grayed out" menu items. The result was Chicago, whose thick verticals make it easy to read even when only every other pixel is displayed.

Chicago is notoriously ugly in any size other than 12 point. In Mac OS 9, Chicago was finally replaced with Charcoal, a similar font which looks rather better in larger font sizes and when anti-aliased.

Many Mac advocates believe that Microsoft chose "Chicago" as the code name for Windows 95 because it was intended to mimic the Mac OS.