“The Jewish people generally speaking waited until the error of the lunar calendar amounted to about a whole month and then inserted an extra month called Veadar, between the months of a Adar and Nisan. A year with this extra month amounted to almost 400 days instead of the usual 354 days of the Jewish lunar calendars.”
Veadar means “and Adar” which can also be called Adar Sheni or “Second Adar” plus Adar Rishon or “First Adar” for the normal 12th Adar on the calendar. When there are two Adars they can also be called Adar Aleph (Adar 1) and Adar Bet (Adar 2).
The long stretches between the years having the extra months were the Dearth Veadar .....