The phrase "bittersweet moment" refers to a situation or experience that evokes both positive and negative emotions simultaneously. For instance, you might feel happiness and sadness at the same time. This could occur during events such as graduation, where you're joyful about the accomplishment but also sad to leave behind close friends and familiar surroundings.
The term "bittersweet" is a compound word formed from "bitter," which comes from the Old English "bitter" meaning "having a sharp taste," and "sweet," originating from the Old English "swete," meaning "having a pleasant taste." The concept combines the two opposing tastes to illustrate mixed feelings.