"And Abraham again took a wife" (Genesis 25:1). Wherever it is said "and he added [vayosef]," there was an addition there. For at first two sons were born to him, Ishmael and Isaac, and here four sons were born to him. "And her name was Keturah." Why was her name called Keturah? Because she became like the scent of the incense [ketoret]. Another interpretation: because she was bound up [niktərah] in purity, for no other man ever came upon her, and Abraham found her bound [keshurah]; therefore she was called Keturah.
Why Abraham's Wife Keturah Was Named for Incense and Purity
Midrash Aggadah, Genesis 25:1
ויוסף אברהם ויקח אשה. כל מקום שנאמר ויוסף היה שם תוספת, שמתחלה נולדו לו שני בנים ישמעאל ויצחק, וכאן נולדו לו ארבעה בנים: ושמה קטורה. למה נקרא שמה קטורה, שנעשית כריח הקטורות, ד"א שנקטרה בטהרה, שלא בא אדם אחר עליה מעולם, ומצאה אברהם קשורה, לכך נקראת קטורה: