An associative learning process in which the individual has learned that a given stimulus arouses anxiety/fear.
Reiss, S., Peterson, R. A., Gursky, D. M., & McNally, R. J. (1986). Anxiety sensitivity, anxiety frequency and the prediction of fearfulness. Behaviour Research and Therapy, 24(1), 1-8. doi: 10.1016/0005-7967(86)90143-9.