I just want to spend a few minutes to clarify the difference between these terms. There is some confusion out there and as soon as three people repeat the wrong thing it becomes truth.
I’m going to add some updates to this in bold. The purpose of this post was not so much to define meito but to disambiguate between meibutsu and meito as people were conflating them. One online statement that prompted this post was that a sword that had a name would be illegal to export from Japan. That idea took several independent facts and conflated them (that meito means it is meibutsu, and then that meibutsu were illegal to export, which they are not). In trying to clear that up I may have introduced some more confusion on how to interpret meito, so I’ll add more to the end.
Meito: This is a sword with a name (a Gō 号). There is an implication of above average quality or importance or fame associated with the use of this word. Any sword can have a name. What we primarily care about this though is a historical name, that is, the blade in question had a name during the Edo period. An example here is the Sunnokina Masamune. It is simply a Masamune that came down through the Edo period with a nickname. This term meito is also used to casually indicate swords of great quality and importance, that may in fact have no name (but we imagine they would be worthy of one). There are no legal restrictions on ownership or movement of meito. Sometimes the NBTHK will indicate a name for a sword in the Juyo or Tokuju papers, in other times it can be discovered through other books or often on the sayagaki where an authority has preserved the name. Sometimes the name comes down with no history at all.
Meibutsu: These are special meito that are on the list of the Kyoho Meibutsu Cho, (享保名物長) the most famous swords in the country in Edo period Japan. These also have no restrictions on ownership. However, many of these also happen to be Juyo Bijutsuhin, Juyo Bunkazai and Kokuho and as a result of that kind of status, would be illegal to export from Japan. An example of this would be the Kanze Masamune.
All meibutsu are meito but not all meito are meibutsu.
Utsushi also sometimes take the name of the source blace, for instance Kunihiro coped the Yamanba-giri Chogi, the resulting copy also became famous over the years and so took on a similar name to the original, becoming the Yamanba-giri Kunihiro. Both blades in this situation are meito, but not meibutsu, and both are Juyo Bunkazai making them illegal to export from Japan.
Elucidating on meito: it may be too strong a word to refer to a named sword (a sword with a Gō 号) that doesn’t have sufficient fame.
Informally, calling a sword with or without a Gō a meito is drawing on some of the aura of those blades in the meibutsucho.
So we need I think to look at this as kind of a spectrum. On one end we have swords with Gō 号 (named swords), and on the other end we have meibutsu 名物 (famous named swords in the meibutsucho). In the middle of the spectrum they start to be meito (名刀) and this extends over all of the meibutsucho. Another example meito would be the Nakigitsune Kuniyoshi which is a famous sword with a name but not on the meibutsucho.
It still serves that not all meito are meibutsu though all meibutsu are meito. But I should be adding that not all swords with go are meito though all conventional meito have go.
To this we add the informal use of meito and can add the fine print that depending on context, meito may not have a name but is a term reflecting the high quality of a particular piece making it something in a class with blades that are otherwise famous.