A Lazarus species (or taxon) is "an animal or plant which disappears from the fossil record, presumably because it is extinct, and then reappears. In some cases, an animal which is thought to be totally extinct may be spotted and described alive, sometimes millions of years after the last fossil evidence of the species has vanished. This illustrates the unreliability of the fossil record; a special set of circumstances must come together for a fossil to form, making fossilization extremely rare." The use of the term "Lazarus" is a reference to the New Testament story in which Jesus raises Lazarus from the dead.
"Every once in a while, a species that hasn't gone extinct will disappear from the fossil record," writes Tracy V. Wilson at HowStuffWorks.com. "Sometimes, this is because a life form has evolved into a new species—this is known as pseudoextinction. Life forms can also disappear from the fossil record and reappear later. These Lazarus species may have experienced a dip in population, or they may not have died in conditions that lead to fossilization."
My favorite example of a Lazarus taxon is the coelacanth More about the coelacanth: "Three days before Christmas, 1938, in the South African coastal town of East London, Marjorie Courtenay-Latimer, the young, black-eyed curator of the local natural history museum, got a phone call that would turn her world upside down and ultimately make her name known internationally. The events that followed that call sparked a series of urgent letters between Courtenay-Latimer and J.L.B. Smith, a chemistry professor and amateur ichthyologist at Rhodes University in the nearby town of Grahamstown. The letters chronicle the discovery she made, and he confirmed, of a creature thought extinct for at least 66 million years."
For the more visually minded, here's a video on Lazarus taxa (yeah, that's plural for taxon).