Ancient Greek for "know thyself". A widely held
maxim in Hellenistic times, inscribed on the entrance to
the temple of Apollo in Delphi among other places.
Mythologically attributed to the god Apollo, who was
asked "What is best for man?" by Chilon of Sparta.
The quote has
been attributed to various real people, including Pythagoras, but its actual origins are obscure,
and even its very meaning is quite debatable.
(Which, of course, may well be the entire point.)
Also a title of a poem by Ralph Waldo Emerson.
Gnothi and gnosis are both derived
from the Greek verb gignôskein, to know.