What is Medium Term Note?

Legal Definition
A medium-term note (MTN) is a debt note that usually matures (is paid back) in 5–10 years, but the term may be less than one year or as long as 100 years. They can be issued on a fixed or floating coupon basis. Floating rate medium-term notes can be as simple as paying the holder a coupon linked to Euribor +/- basis points or can be more complex structured notes linked, for example, to swap rates, treasuries, indices, etc. When they are issued to investors outside the US, they are called "Euro Medium Term Notes" (EMTN). Issuance of MTNs to investors based in the US requires a separate US MTN program.

MTNs can be issued with a fixed maturity date (noncallable) or can be issued with embedded call or put options and triggers where the notes will redeem early based on certain parameters. MTNs are most commonly issued as senior, unsecured debt of investment grade credit rated entities which have fixed rates. MTNs offer more flexibility to the issuer and investor both in terms of structure and documentation
-- Wikipedia
Legal Definition
When an underwriter arranges for a security to be issued.