The Milky Way - image source: http://www.digitalskyllc.com

The Milky Way – image source: http://www.digitalskyllc.com

1. A principle (noun only) is a basic law or truth, on which we base other actions. For example:
Gravity is the principle (main or basic law) that holds our galaxy, and the rest of the universe, in an ever-changing balance.
The principle of supply and demand rules the world of commerce.
The principle of hydraulics allows force to be transmitted through pipes, using liquids to push pistons in order to move remote parts. This can be seen in well-known examples like brake systems and earth-moving machines.
2. A principle is a personal code of conduct. For example:
She is a very principled (ethical) person and her strong beliefs help her to decide how to act in any given situation.
On principle, I avoid violence at all costs, because I believe that one violent act only creates another.
3. A principle is a kind of jacket.

Principle Jacket: Assymetrical Zipper keeps ch...

Principle Jacket: Assymetrical Zipper keeps chafing at bay! (Photo credit: lululemon athletica)

Principle has these basic synonyms (there are more):

1. assumption, belief, creed, criterion, doctrine, dogma, belief, law, notion, rule, standard
2. conscience, honesty,

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1. A principal is the, for example, the head or most important person in an organisation. For example:
Principal (noun) is synonymous with head master/mistress.

2. Principal (adj) also means the main or most important part. For example:
The principal actor in a play plays the most important part.
The principal ingredient of bread is grain.
The principal component of butter is cream.
The principal cause of war is resources.

3. Principal is also the capital part of a loan, as separate from interest.