Here's why:
* Concrete nouns refer to physical things you can touch, see, hear, smell, or taste. Examples: table, dog, book, rain.
* Abstract nouns refer to ideas, concepts, qualities, or feelings. They represent things you can't experience with your senses. Examples: love, happiness, time, justice.
An omen is a sign or event that is believed to foretell a future event. You can't touch, see, or hear an omen itself – it's an idea or interpretation of something else.