When discussing units of pressure, the abbreviation Ba stands for barads which is the metric, cgs (non-SI) unit of pressure that is equal to 0.1 Pa.
In science, the abbreviation “Ba” may also stand for the element barium. That is probably the more common definition, because the unit barad is outdated and confusing.
In fact, the barad is so unstandardized that you may also see the barad called: baryd, baryed, barye, barrie, barie, or bary.
Since the barad was used in the cgs system, it is defined as 1 dyne per square centimeter.
(The dyne is the cgs unit of force, equal to 10-5 newtons.)
1 Ba = 1 dyne/cm2
If you put the barad in terms of pascals (the SI unit of pressure), it works out to 1 Ba = 0.1 Pa.
This article was a definition of Ba. If you are interested in all the different, strange units that are used to measure pressure, I wrote an entire article explaining what each unit is, and how to convert between them. Or, if there is a unit you are especially interested in, you can check out one of the other unit definitions I’ve written.
Other Pressure Measurements:
mbar or bar
psi or ksi