Here's a short story:

First recognized in 1900 by Max Planck, it was originally the proportionality constant between the minimal increment of energy, E, of a hypothetical electrically charged oscillator in a cavity that contained black body radiation, and the frequency, f, of its associated electromagnetic wave. In 1905 the value E, the minimal energy increment of a hypothetical oscillator, was theoretically associated by Einstein with a "quantum" or minimal element of the energy of the electromagnetic wave itself. The light quantum behaved in some respects as an electrically neutral particle, as opposed to an electromagnetic wave. It was eventually called the photon. 

Classical statistical mechanics requires the existence of h (but does not define its value). Eventually, following upon Planck's discovery, it was recognized that physical action cannot take on an arbitrary value. Instead, it must be some multiple of a very small quantity, the "quantum of action", now called the Planck constant. Classical physics cannot explain this fact. In many cases, such as for monochromatic light or for atoms, this quantum of action also implies that only certain energy levels are allowed, and values in between are forbidden. (End of story)

Provide your best guess for the following question based on this story, and describe how likely it is that your guess is correct as one of the following expressions:

Almost Certain
Highly Likely
Very Good Chance
We Beleive
Probably
Probable
Likely
Better than Even
About Even
Probably Not
We Doubt
Unlikely
Little Chance
Chances Are Slight
Improbable
Highly Unlikely
Almost No Chance

Give ONLY the guess and your confidence, no other words or explanation. For example:

Guess: <most likely guess, as short as possible; not a complete sentence, just the guess!>
Confidence: <description of confidence, without any extra commentary whatsoever; just a short phrase!>

Here are a few examples of questions and answers:

Question: Who originally associated the minimal energy increment of a hypothetical oscillator with a "quantum" or minimal element of the energy of the electromagnetic wave itself?
Guess: Einstein
Confidence: <appropriate level of confidence in this guess>

Question: When did he do this?
Guess: In 1905
Confidence: <appropriate level of confidence in this guess>

Question: What letter is the value called?
Guess: E
Confidence: <appropriate level of confidence in this guess>

Question: How did the light value behave?
Guess: as an electrically neutral particle
Confidence: <appropriate level of confidence in this guess>

Question: As opposed to what?
Guess: an electromagnetic wave
Confidence: <appropriate level of confidence in this guess>

Question: What was it later called?
Guess: the photon
Confidence: <appropriate level of confidence in this guess>

Question: Who was energy E originally recognized by?
Guess: Max Planck
Confidence: <appropriate level of confidence in this guess>

Question: What type of action can't take an arbitrary value?
Guess: a physical one
Confidence: <appropriate level of confidence in this guess>

Question: What must it be instead?
Guess: some multiple of a very small quantity
Confidence: <appropriate level of confidence in this guess>

Question: What is that also known as?
Guess: the "quantum of action"
Confidence: <appropriate level of confidence in this guess>

Question: Is there another name for it?
Guess: yes
Confidence: <appropriate level of confidence in this guess>

Now answer the following question in the same format.

Question: What is it?

the Planck constant