וַתֹּֽאמֶר־לָ֥הּ בַּת־פַּרְעֹ֖ה לֵ֑כִי וַתֵּ֙לֶךְ֙ הָֽעַלְמָ֔ה וַתִּקְרָ֖א אֶת־אֵ֥ם הַיָּֽלֶד׃
Da sprach zu ihr die Tochter Pharaos: Gehe! Und das Mädchen ging und holte die Mutter des Kindes.
Rashi on Exodus
ותלך העלמה AND THE YOUNG WOMAN WENT — she went eagerly and with vigour (עלמות) like a young man (עלם) (cf. Sotah 12b).
Chizkuni
ותלך העלמה, “the young lady went, etc.” The reason why the Torah calls her by the description עלמה, is because she concealed the fact that she was the baby’s sister.