It's usually the flavor of glutamate, which is an amino acid found in foods such as meat, dairy products, fish and vegetables. An avocado definitely doesn't fit into any of the other categories, and umami is the closest category I could find that precisely encompasses the very mild flavor of an avocado. Some sources suggest that steaming or cooking avocados makes them taste like hard-boiled eggs, while others have noticed the egg-like flavor in raw avocados (which has also been my own experience). Strictly speaking, a large berry containing only one seed, the avocado is the fruit of the avocado tree, native to Mexico, and is the main ingredient in guacamole.
Considered a “superfood” because of its highly nutritious fat content, including oleic acid, its high levels of vitamin B, C, E, K and potassium, and its high umami content, avocado has become extremely popular as a staple food in Western vegetarian diets. For macronutrient ratios, avocado is lighter in protein, heavier in carbohydrates, and heavier in fat compared to peanut butter per calorie.