Adverb
Verb (する)
Onomatopoeic
Usually written in kana
1. motionlessly (e.g. stand, wait); (be) still
じーっと is more emphatic. 凝乎と is rare.
2. fixedly (e.g. gaze, stare); intently (e.g. listen, think)
3. patiently (endure); stoically
4. firmly (e.g. hold); restrained