Discretion is knowing what not to say. The colleague who hears something private and never mentions it again, the doctor who does not talk about patients at parties.
The second sense is about power rather than silence: your discretion is your freedom to decide. A judge has discretion over the sentence, entry is at the manager's discretion, and 'use your discretion' means nobody is going to hand you a rule. Watch the two adjectives. Discreet, careful and quiet, is the one that belongs to discretion; discrete, with the e's apart, means separate and belongs to another word entirely. The old saying goes that discretion is the better part of valour — knowing when not to fight is a kind of courage.
1nountactGood judgement about what should be kept private and what should be said.
“She handled the whole complaint with discretion.”
2noundiscretionThe freedom or authority to decide something according to your own judgement.
“Payment of the bonus is at the director's discretion.”
discretion is a quality of judgement.
Discretion goes back through Old French and Latin discrētiō to the verb discernō, 'to separate, to tell apart'. That is the whole idea: a person with discretion tells one thing from another — what may be said from what may not, one case from the next. The deeper root, Proto-Indo-European *krey-, is about sifting and separating.
Proto-Indo-European
*krey-
Latin
discrētiō
Old French
discretion
Middle English
discrecioun
English, today
discretion
From Middle English discrecioun, from Old French discretion, from Latin discrētiō, from Latin discerno. Equivalent to discreet + ion.
Etymology from Wiktionary, CC BY-SA