When Jill experienced her first panic attack, she felt as if she were outside of herself, watching herself struggle to catch her breath. Jill's sense of not being part of herself is one of the symptoms of a panic attack known as

depersonalization.  

Explanation:

Depersonalization: In psychology, the term "depersonalization" is described as an individual's sense of experiencing his or her own feelings, behavior, and thoughts from a "dreamlike distance". An individual who is experiencing depersonalization often feels emotionally numb, a sense of not being part of one's own body, not able to control one's actions or words, detached from basic sensations, for example, libido, touch, hunger, and thirst, etc.

In the question above, Jill's symptoms of a panic attack is known as depersonalization.