The BAI is a (Beck, A. T., Epstein, N., Brown, G., & Steer, R. A. (1988). An inventory for measuring clinical anxiety: Psychometric properties. Journal of Consulting and Clinical Psychology, 56(6), 893–897. https://doi.org/10.1037/0022-006X.56.6.893) is a 21 item self-report questionnaire, to a large extent the sibling of the Beck Depression Inventory (BDI, BDI-I and BDI-II). Its development was very closely linked with the development of Beckian cognitive behavioural therapy (CBT). It is a commercial measure owned by Pearson, see here.