At peace

the normal, nonwarring condition of a nation, group of nations, or the world.
(often initial capital letter) an agreement or treaty between warring or antagonistic nations, groups, etc., to end hostilities and abstain from further fighting or antagonism:
the Peace of Ryswick.
a state of mutual harmony between people or groups, especially in personal relations:
Try to live in peace with your neighbors.
the normal freedom from civil commotion and violence of a community; public order and security:
He was arrested for being drunk and disturbing the peace.
cessation of or freedom from any strife or dissension.
freedom of the mind from annoyance, distraction, anxiety, an obsession, etc.; tranquillity; serenity.
a state of tranquillity or serenity:
May he rest in peace.
a state or condition conducive to, proceeding from, or characterized by tranquillity:
the peace of a mountain resort.
silence; stillness:
The cawing of a crow broke the afternoon’s peace.
(initial capital letter, italics) a comedy (421 b.c.) by Aristophanes.
(used to express greeting or farewell or to request quietness or silence).
Obsolete. to be or become silent.
at peace,

in a state or relationship of nonbelligerence or concord; not at war.
untroubled; tranquil; content.
deceased.

hold / keep one’s peace, to refrain from or cease speaking; keep silent:
He told her to hold her peace until he had finished.
keep the peace, to maintain order; cause to refrain from creating a disturbance:
Several officers of the law were on hand to keep the peace.
make one’s peace with, to become reconciled with:
He repaired the fence he had broken and made his peace with the neighbor on whose property it stood.
make peace, to ask for or arrange a cessation of hostilities or antagonism.
noun

the state existing during the absence of war
(as modifier): peace negotiations

(modifier) denoting a person or thing symbolizing support for international peace: peace women
(often capital) a treaty marking the end of a war
a state of harmony between people or groups; freedom from strife
law and order within a state; absence of violence or other disturbance: a breach of the peace
absence of mental anxiety (often in the phrase peace of mind)
a state of stillness, silence, or serenity
at peace

in a state of harmony or friendship
in a state of serenity
dead: the old lady is at peace now

hold one’s peace, keep one’s peace, to keep silent
keep the peace, to maintain or refrain from disturbing law and order
make one’s peace with, to become reconciled with
make peace, to bring hostilities to an end
verb
(intransitive) (mainly obsolete) to be or become silent or still
n.

mid-12c., “freedom from civil disorder,” from Anglo-French pes, Old French pais “peace, reconciliation, silence, permission” (11c., Modern French paix), from Latin pacem (nominative pax) “compact, agreement, treaty of peace, tranquility, absence of war” (source of Provençal patz, Spanish paz, Italian pace), from PIE *pag-/*pak- “fasten,” related to pacisci “to covenant or agree” (see pact).

Replaced Old English frið, also sibb, which also meant “happiness.” Modern spelling is 1500s, reflecting vowel shift. Sense in peace of mind is from c.1200. Used in various greetings from c.1300, from Biblical Latin pax, Greek eirene, which were used by translators to render Hebrew shalom, properly “safety, welfare, prosperity.”

Sense of “quiet” is attested by 1300; meaning “absence or cessation of war or hostility” is attested from c.1300. As a type of hybrid tea rose (developed 1939 in France by Francois Meilland), so called from 1944. Native American peace pipe is first recorded 1760. Peace-officer attested from 1714. Peace offering is from 1530s. Phrase peace with honor first recorded 1607 (in “Coriolanus”). The U.S. Peace Corps was set up March 1, 1962. Peace sign, both the hand gesture and the graphic, attested from 1968.
In a state of agreement or friendliness, not at strife or war; also, in a state of inner harmony or quiet. For example, Whatever their disagreements, Mexico and Belize have remained at peace, or In his last illness he seemed finally to be at peace with himself. [ 1300s ]
In addition to the idiom beginning with
peace
also see:

at peace
hold one’s tongue (peace)
keep the peace
leave someone in peace
make one’s peace with
make peace