Archaic words

At least, I think that's what they are.
I'm reading Fletcher Pratt's _The Well of the Unicorn_, and have
stumbled over "deserion", "deese", and "tercia". From context, I
think that all of them are military terms. Maybe
Searching on-line let me know that the first is obviously a misspelled
"desertion" and the third is a given name in Spanish.
Does anybody here know what these words really mean?
<https://www.isfdb.org/cgi-bin/title.cgi?15607>
--
Michael F. Stemper
87.3% of all statistics are made up by the person giving them.