All 3 Uses of

serene
in
Pride and Prejudice

But in all, and in almost every line of each, there was a want of that cheerfulness which had been used to characterise her style, and which, proceeding from the serenity of a mind at ease with itself and kindly disposed towards everyone, had been scarcely ever clouded.†
p. 184.3

serenity = untroubled calm

But I shall not scruple to assert, that the serenity of your sister's countenance and air was such as might have given the most acute observer a conviction that, however amiable her temper, her heart was not likely to be easily touched.†
p. 192.7 *

Mrs. Bennet was restored to her usual querulous serenity; and, by the middle of June, Kitty was so much recovered as to be able to enter Meryton without tears; an event of such happy promise as to make Elizabeth hope that by the following Christmas she might be so tolerably reasonable as not to mention an officer above once a day, unless, by some cruel and malicious arrangement at the War Office, another regiment should be quartered in Meryton.†
p. 230.4

Definition:

calm and untroubled