gerund: Gerunds are verb forms used as nouns. They are similar to participles in that they both use the same kinds of modifiers and complements. The key to understanding gerunds is knowing that gerunds act like nouns. Where you can place a noun, you place a gerund.
Running is good exercise. (Gerund used as a subject noun)
Walking to school is a good idea.
My hobby is hiking. (Gerund used as a direct object)
I reached him by calling his office. (Gerund used as the object of a preposition).
Note: the difference between:
My hobby is hiking.
John is hiking.
Litmus tests for gerunds: Can you put a noun there?
Tennis is good exercise.
The wheel was a good idea.
My hobby is chess.
I reached him by email.