adverb
at top speed; at full speed; as fast as one can
Describes moving or doing something very quickly, often with a sense of urgency or single-mindedness. Can be used for running, working, or other actions.
彼は一散に走り出した。
He started running at top speed.
一散に仕事を片付けた。
I finished the work as fast as I could.
全速力 is a noun meaning 'full speed' or 'maximum speed', often used in phrases like 全速力で走る. 一散に is an adverb that emphasizes the manner of doing something quickly and with focused effort.
一目散に also means 'at full speed' but often implies fleeing or rushing away in a straight line. 一散に is more general and can be used for any fast, focused action.
The exact derivation is uncertain. The kanji 一 (one) and 散 (scatter) may suggest a single-minded scattering or rushing, but this is not a well-documented historical etymology.