Greek for "everything flows", implying that nothing is permanent but is always in flux, or changing.
The expression is attributed to Heraclitus, a Greek philosopher who lived around the same time as the Buddha, and amazingly came up with many similar ideas.
I have heard panta rhei expressed as "The only thing that does not change is the fact that everything changes." I am not sure, though, whether those are Heraclitus' exact words or a later paraphrase.