Relating to the writing of letters.
Originally from Latin, and gives its name to literary works written as a series of letters or messages.
“It’s one of my favourite epistolary novels!”
“We keep an epistolary relationship, even after all these years.”
With regard to, or at the level of, orders in a taxonomic system; according to an ordinal number, system, etc.; on an ordinal scale; with regard to order.
Mid 19th century; earliest use found in The Edinburgh Review. From ordinal + -ly.