Answer:
A gerund
Explanation:
A gerund is a type of verbal that acts as a noun in a sentence, thus, it can also act as a subject. This verbal is formed with the root of a verb + ing, which it's the present participle form. For example: Think + ing = Thinking. Here are some examples:
Skydiving is not that easy.
The gerund Skydiving acts as a noun: It refers to the sport or activity of jumping from an aircraft and performing acrobatic maneuvers in the air under free fall before landing by parachute. And it is also the subject as is the main "thing" being described.
Swimming is a great cardiovascular exercise.
The gerund Swimming acts as a noun as it refers to the skill of swimming or the sport of swimming itself, and it's the subject as well as it is what is being dealt with in the sentence.
