1. continuing forever without end

e.g. there are infinitely many possibilities

Synonym: infinitely

2. with unflagging resolve

e.g. dance inspires him ceaselessly to strive higher and higher toward the shining pinnacle of perfection that is the goal of every artiste

Synonym: ceaselesslyincessantlyunceasinglyunendinglycontinuously

3. all the time
seemingly without stopping

e.g. a theological student with whom I argued interminably
her nagging went on endlessly

Synonym: interminably

4. (spatial sense) without bounds

e.g. the Nubian desert seemed to stretch out before them endlessly