"fear itself", is the answer that comes to mind most frequently because of the famous quotation. My opinion of the definition of fear is the sensation of mild to extreme anxiety at the thought of an impending, unpleasant situation. So fearing fear itself is basically being anxious about being anxious. While this makes sense in-and-of-itself, as a definition of the root of fear, it is basically circular logic and doesn't really provide an accurate answer.
My opinion of the root of fear is simply the fear of death. For instance take this conversation. . ."I have a fear of heights", "Oh? What is it exactly about heights that scares you?", "I don't know, I just get up there, look down and get terrified I'm going to fall", "So it would be more accurate to say that you have a fear of falling?", "yes", "ok, so what is it about falling that scares you so much?", "Well, hitting the ground I guess", "So again, to be even more accurate your fear is not really of heights, or falling, it's of hitting the ground?","yes", "and what is it about hitting the ground that you fear?". . . . ."Dying".
I have found that this "pairing down" logic works with the majority of fears. Because the conclusion of each of these "fear cycles" appears to be the "fear of death", I would have to say that this would be the root of fear.
Fearful
Fear is the root word.
The root meaning of claustrophobia comes from the Latin word "claustrum" meaning enclosure or confined space, and the Greek word "phobos" meaning fear. Combined, it refers to the fear of enclosed spaces.
Trepidation=dread,fear, sudden fear, fear of what may not happen;
The Greek root phobos means fear i.e aquaphobia, the fear of water or just plain phobia.
Fear
fear
Any of the phobias use this root (arachnophobia, agoraphobia, etc.).
The Greek root of "homophobia" is "homo," which means "same" or "alike," and "phobia," which refers to an irrational fear. Homophobia is the fear or hatred of homosexuals or homosexuality.
The term based on the Greek root word for water and fear is "hydrophobia." It is used to describe an irrational fear of water or of being in water.
Radical Fright
The Greek root word for fear is "phobos." It is commonly used in English to form words related to fears and phobias, such as "phobia" and "phobic."