A sign or indication that something is about to happen, often used for both positive and negative events. Common in written and formal contexts.
I felt a sign of an earthquake.
Signs of economic recovery can be seen.
予兆 refers to an objective sign or omen that something will happen, while 予感 is the subjective feeling itself.
フラグ is a casual, often humorous term used mainly online for a foreshadowing moment, whereas 予兆 is a neutral, everyday word for any early sign of an upcoming event.
予兆 just means a sign or forewarning of something to come (good or bad), while 吉兆 specifically means a sign of good fortune.
Sino-Japanese compound: 予 (beforehand) + 兆 (sign, omen).