Hi John,
Not only have some Latin words taken on different meaning over the centuries,
but there are many Latin words used today that Vergil, Pliny, and Cicero
never knew.  Over a period of a few hundred years, the "classical Latin" that
these fellows spoke and wrote evolved into what is called Late Latin, with
the introduction of a number of new words and phrases.  Then, over the next
few hundred years (more or less - I am not a historian), this form of the
language evolved into Medieval Latin, which was used in matters of state,
law, religion, and business throughout the Middle Ages.  Again, many new
terms were added during this period, though Latin as a vernacular language
was on the way out, as the various Romance languages developed.  Then, around
the 15th or 16th century, Latin again came into prominence with the new
emphasis on science.  Beginning here we have what is known as "new Latin",
which has added countless new words, primarily of a scientific nature.  Some
of the new words described things that the ancient Romans didn't know
existed, and therefore had no word for - like paramecium and ameba.  As the
naming of organisms got underway, many names were drawn from classical Latin
- like Conus, Nautilus, Pecten, and Murex.  But many new words were created
also, which had no meaning in classical Latin, like Phalium, Cerithium,
Haliotis, Dosinia, Physa, Acmaea, Mactra, Pinctada, etc.  These names don't
mean anything in Latin except as names for the mollusks they describe, and
the three gentlemen mentioned above would not have recognized any of them.
Regards,
Paul M.