Tissot Le Locle Chronometer
From the very first clocks to the invention of the quartz crystal, a preoccupation with producing ever more accurate timepieces dominated the history of watchmaking. But after quartz, which is more accurate than any mechanical mechanism, this largely died out. Keeping the tradition going though is the COSC chronometer designation, a mark of stringent accuracy under duress - and in 2012, Tissot's Le Locle Chronometre was pronounced by the Concours International de Chronometrie to be the most accurate mechanical watch of the year.
So I guess the first question here is what exactly is a chronometer? The short answer is an extremely accurate mechanical timekeeping device - clock, watch, or otherwise. But the long answer is much more complicated than that, and much more interesting.
This answer takes us back to the early eighteenth century, where many of watchmaking's greatest stories begin. In 1714, English horologist Jeremy Thacker managed to create a clock completely sealed in a vacuum chamber, eliminating any air friction. He called this creation a chronometer, giving it a new name to celebrate its improved accuracy. But, as you might have guessed, sealing clocks in vacuum chambers isn't the most practical solution to the accuracy problem in practice.
Most innovations in timekeeping during this period were driven by the search for more accurate navigational tools. And thus, the marine chronometer was born. John Harrison invented the marine chronometer sometime around 1730, which was the first clock accurate enough to ensure proper calculations of longitude during extended sea voyages. This was absolutely necessary for an empire like Britain, and it has been argued more than once that without such an invention the empire would not have been able to sustain itself for so long.
Setting aside these historical antecedents though, chronometers these days are mechanical watches that meet certain standards of accuracy under simulated wear conditions. Within Switzerland there is COSC (Controle Officiel Suisse Des Chronometres) which must certify any timepiece bearing the word "chronometre" on it anywhere - only about 3% of all Swiss watches receive the certification. This protects the term's technical meaning and keeps it out of the hands of the marketing departments. Elsewhere the term is less tightly guarded, though generally it represents some amount of increased accuracy testing.