Doubt is based on the Latin root word "doub" which means two, as in double.
In German, the word for doubt is "zweifel", which is also based on the root word for two- zwei.
So, doubt is to be of two minds.
One mind stays connected to who you really, full of abundance and clarity, the other mind connects more or less to a sense of separation and lack. A mind full of doubt is full of thoughts of lack and uncertainty, and the mind goes about trying to figure out how to fill the gap.
Self-doubt is believing more in separation and emptiness than the fullness of who you really are.
So to be without a doubt is to be single minded, with faith or confident.
There is a clear difference in the way it feels to be aligned with faith and abundance or aligned with lack and doubt. Doubt always separates you from your completed potentials, even in thought. While you embrace the feeling of single-minded focus, your perceptions open to conditions relative to that focus.