summary

First recorded in 1400–50; late Middle English, from Latin summārium, equivalent to summ(a) “sum” + -ārium noun suffix; see sum, -ary

1. Summary, brief, digest, synopsis are terms for a short version of a longer work. A summary is a brief statement or restatement of main points, especially as a conclusion to a work: a summary of a chapter. A brief is a detailed outline, by heads and subheads, of a discourse (usually legal) to be completed: a brief for an argument. A digest is an abridgment of an article, book, etc., or an organized arrangement of material under heads and titles: a digest of a popular novel; a digest of Roman law. A synopsis is usually a compressed statement of the plot of a novel, play, etc.: a synopsis of Hamlet.

The English noun summary comes straight from the Latin neuter noun summārium “abridgment, abstract, epitome,” an extremely rare word used only once in the surviving Latin literature by the Roman author, tragedian, statesman, and Stoic philosopher Seneca (the Younger) in one of his Moral Letters to Lucilius (39), in which he complains “…what is now commonly called a ‘breviary’ [ breviārium ] was called, in the good old days, when we used to speak Latin, a ‘summary’ [ summārium ]." (Complaints about the terrible state of the language are nothing new.)
Summārium is a compound of adjective summus “highest, topmost, top” and the noun suffix -ārium. ( Summa, the feminine of summus used as a noun, in mathematics and accounting means “sum, total”: The Romans added their numbers from the bottom up and wrote the total in summā “on the top.”)
Medieval Latin has the adjective summārius “abbreviated, summary,” which was borrowed into Middle English in the 15th century.The adjectival meaning “relating to legal proceedings conducted without certain required formalities” is recorded about 1765, though the corresponding meaning of the adverb summarily appears much earlier.