What is inspiration?  Here is a start to Answering the Question:

The Technical Definition

1. The physically act of breathing in or inhaling. The drawing of air into the lungs.
2. A sudden idea.
3. The feeling itself  within the body of elevated energy and enthusiasm
4. The sources of inspiration is also called an inspiration. Someone or something that causes this state.
5. A product of inspiration, such as an idea,  piece of art, an invention, a business,  etc.

Newton sits under the apple tree. An apple falls and hits him on the head. He suddenly becomes excited and full of energy as an idea for the theory of gravity comes to him. He writes down the idea in his notebook 

In this story, the apple can be called an inspiration, since it caused the enthusiasm and the sudden idea.  The sudden idea is an inspiration and so is his state of enthusiasm. Finally, his notes are the product of the inspiration and can themselves be called an inspiration.  So, the word inspiration seems to have a lot of different but related meaning. Looks to me like the English language is not very exact.

Defintinon from Bartleby.com, The American Heritage Dictionary of the English Language

1.

    a. Stimulation of the mind or emotions to a high level of feeling or activity.

    b. The condition of being so stimulated.

2. An agency, such as a person or work of art, that moves the intellect or emotions or prompts action or invention.

3. Something, such as a sudden creative act or idea, that is inspired.

4. The quality of inspiring or exalting: a painting full of inspiration.

5. Divine guidance or influence exerted directly on the mind and soul of humankind.

6. The act of drawing in, especially the inhalation of air into the lungs.

To see the definitions given by many other dictionaries, see the following url.
http://www.humanityquest.com/topic/Definitions/index.asp?theme1=inspiration

Various peoples comments

Inspiration is an awakening, a quickening of all man's faculties, and it is manifested in all high artistic achievements. Giacomo Puccini

 

Inspiration is a fragile thing... just a breeze, touching the green foliage of a city park, just a whisper from the soul of a friend. Just a line of verse clipped from some book. Inspiration... who can say where it is born, and why it leaves us? Who can tell the reasons for its being or not being? Only this... I can think. Inspiration comes from the Heart of Heaven to give the lift of wings, and the breath of divine music to those of us who are earthbound.   Margaret Sangster 

 

Inspiration is a sustainable internal glow which PULLS you forward.  Giuseppe Mazzini

 

Inspiration is a guest who does not like to visit lazy people. Tchaikovsky

...inspiration is as necessary to the life of goodness, holiness and happiness as perpetual respiration is necessary to animal life. Andrew Bonar Law