Final answer:
The word 'myriad' used to mean exactly 10,000 in ancient Greece, but now refers to any large, indefinite number.
Step-by-step explanation:
The six-letter word that originally meant exactly 10,000 in ancient Greece is myriad. The term myriad is used today to describe any large, indefinite number but was used by ancient Greeks to denote a precise count of 10,000. This usage is reflected in historical documentation and the structure of the Greek language during that era, where numbers played a significant role both in mathematics and in various aspects of Greek culture, including their mythological and historical narratives.