Irritation is a reaction of redness, pain, burning or irritation caused by the contact of a substance or an external factor with a part of the body (usually the skin, eyes, respiratory tract or digestive tract). Irritation is usually a transient response and subsides over time when contact with the causative substance is discontinued.

Some common examples of irritation are:

1. *Skin Irritation:* A skin rash, itching or burning sensation may occur due to chemicals, cleaning products, certain herbs (for example, poison ivy) or excessive friction.
 
2. *Eye Irritation:* When smoke, dust, chemicals or foreign objects enter the eye, redness, burning and watering may occur in the eye.

3. *Respiratory Tract Irritation:* Inhaling cigarette smoke, dusts, chemical vapors or allergens may cause burning in the nose, throat or lungs, coughing or difficulty breathing.

Irritation is different from an allergic reaction. An allergic reaction occurs as a result of the immune system showing hypersensitivity to a substance (allergen), while irritation is directly caused by the irritating effect of a substance.