What is the term for an irrational fear of something?

The term for an irrational fear of something is "phobia." This word originates from the Greek term "phobos," meaning fear or dread. Phobias are often disproportionate to the actual danger posed by the object of the fear, leading to significant distress and avoidance behavior. For example, someone with a phobia of spiders might react with extreme fear to even the sight of a picture of a spider, even though it poses no real threat.

The other terms provided do not relate to irrational fears. "Ostomy" refers to a surgical procedure that creates an opening from an area inside the body to the outside, commonly used in relation to bowel surgeries. "Genesis" pertains to the origin or mode of formation of something, often used in biological or conceptual contexts. "Gravida" is a medical term used to denote a woman who is or has been pregnant. Thus, "phobia" is the only choice that accurately describes an irrational fear.