Things that are unceasing or exist without interruption are continuous. For example, the flow of a river, the motion of the planets around the sun, and the heartbeat of a healthy human are continuous because they never pause. Things that occur frequently or recur intermittently are continual. The continual action doesn’t happen ceaselessly, but it does happen regularly. For example, phone calls to a busy office and departures from a bus station are continual because they happen regularly but not in an uninterrupted stream.
These definitions are only usually borne out in real-world usage. The words have not always been differentiated, and they are still often used interchangeably. When continual entered the language around 1400, it meant what continuous means now,1 and frequently occurring didn’t become its primary definition until the 20th century. Continuous entered the language much later—not until the 17th century—and its primary sense has always been uninterrupted or unceasing,2 but examples of its use in place of continual are easy to find.
Continuous works in these sentences because it describes something happening ceaselessly:
Based on these findings, aboriginal Australians would represent one of the oldest continuous populations outside Africa. [Washington Post]
One of the advantages of not having an insurance industry with hundreds of years of continuous history is that you get to make things up as you go along. [Financial Times]
That furious brown water, swirling, foaming, leaping and thundering, represented for Romantics the continuous force of thought. [More Intelligent Life]
And continual works in these sentences because it describes things that occur frequently:
His office reported continual talks with the Department of State on the orphans’ plight. [Post-Gazette]
Now he also understands what it was like for a younger brother to continually get his head shoved into a spiky hedge by a ruthless older sibling. [Press.co.nz]
After six years of continual plot twists, flashbacks and flash forwards, fans of the mysterious drama “Lost” will begin the show’s final season. [The Daily Athenaeum]
But the words are often used interchangeably, so mixing them up is not a serious error. Examples such as these are easy to find:
The US military has continuously imposed pitifully light “punishments” on its soldiers even for the most heinous atrocities. [Guardian]
There is nothing other than love, and the whole of creation is a continual outpouring of divine love. [Huffington Post]