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Gary:

If you can forgive the pun, the "bar" wasn't defined in a vacuum.  In the deprecated centimeter-gram-second system of measurement units, it's a nice, round 1 megadyne per square centimeter.  It was given its unique name because 1 Mdyn/cm² is "close enough" to atmospheric pressure for a lot of applications.  For example, if a pressure gauge on your steam engine reads 20 bar, adding 1 bar to determine absolute pressure is good enough for a lookup in your steam tables.

Touching on what Geoffrey mentioned, if you're looking through sources older than 50 years or so, 1 bar doesn't necessarily equal 100 kPa.  As a practical matter, atmospheric pressure was determined by mercury barometers, and different organizations determined their standard density of mercury under different conditions (typically a variation in temperature and local gravity; see Bowditch tables 32 & 33).  CGPM eventually had to weigh (ha!) in with their own standards:

• Standard gravity of 980.665 cm/s²
• Standard fluid density of 13.5951 g/cm³
• Standard fluid column height of 76.0 cm

CGPM eventually got away from manometric units entirely with the current 101,325 Pa standard (producing a difference in column height of less than 200 nanometers).