Other forms: classified, classifies, classifying
To classify is to sort things into named groups. Classify the samples, classify by size, hard to classify.
The naming is the point: classifying is not just sorting but deciding which categories exist, which is why classification systems are argued over so fiercely in biology and in libraries. To classify a document is to restrict access to it, which is a separate official sense. Note classification, classified; and that classified ads are named from being sorted into categories.
1verbclassifyTo sort things into named groups.
“Librarians classify books by subject so that readers can find them easily.”
2verbrestrictTo restrict access to a document.
“The government classified the report, so it won't be made public for decades.”
classify is a sorting.
Also accepted in games: categorise
French classifier is class plus -ify, from Latin classis, a division of the Roman people. Class and classic come from the same division.
French
classifier
English, today
classify
From French classifier. by surface analysis, class + -ify.
Etymology from Wiktionary, CC BY-SA