The maximum possible speed in the Star Trek universe (not including the original series). Between the 23rd and 24th centuries, according the the Technical Manual, the warp curve was redrawn to be asymptotic, increasing sharply between Warp 9 and reaching infinity at Warp 10.

In other words, Warp 10 is unachievable by any means in the Star Trek universe. "Voyager" episodes that claim anything else are the result of crap writers who don't research their own material.