principle&princial

Both words have a number of meanings, which are listed below:
Principle -
–noun
1. an accepted or professed rule of action or conduct: a person of good moral principles.
2. a fundamental, primary, or general law or truth from which others are derived: the principles of modern physics.
3. a fundamental doctrine or tenet; a distinctive ruling opinion: the principles of the Stoics.
4. principles, a personal or specific basis of conduct or management: to adhere to one's principles; a kindergarten run on modern principles.
5. guiding sense of the requirements and obligations of right conduct: a person of principle.
6. an adopted rule or method for application in action: a working principle for general use.
7. a rule or law exemplified in natural phenomena, the construction or operation of a machine, the working of a system, or the like: the principle of capillary attraction.
8. the method of formation, operation, or procedure exhibited in a given case: a community organized on the patriarchal principle.
9. a determining characteristic of something; essential quality.
10. an originating or actuating agency or force: growth is the principle of life.
11. an actuating agency in the mind or character, as an instinct, faculty, or natural tendency: the principles of human behavior.
12. Chemistry . a constituent of a substance, especially one giving to it some distinctive quality or effect.
13. Obsolete . beginning or commencement.
—Idioms
14. in principle, in essence or substance; fundamentally: to accept a plan in principle.
15. on principle,
a. according to personal rules for right conduct; as a matter of moral principle: He refused on principle to agree to the terms of the treaty.
b. according to a fixed rule, method, or practice: He drank hot milk every night on principle.
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Origin:
1350–1400; Middle English, alteration of Middle French principe or Latin prīncipium, on the analogy of manciple. See principium

Principal -
adjective
1. first or highest in rank, importance, value, etc.; chief; foremost.
2. of, of the nature of, or constituting principal or capital: a principal investment.
3. Geometry . (of an axis of a conic) passing through the foci.
–noun
4. a chief or head.
5. the head or director of a school or, especially in England, a college.
6. a person who takes a leading part in any activity, as a play; chief actor or doer.
7. the first player of a division of instruments in an orchestra (excepting the leader of the first violins).
8. something of principal or chief importance.
9. Law .
a. a person who authorizes another, as an agent, to represent him or her.
b. a person directly responsible for a crime, either as an actual perpetrator or as an abettor present at its commission. Compare accessory ( def. 3 ) .
10. a person primarily liable for an obligation, in contrast with an endorser, or the like.
11. the main body of an estate, or the like, as distinguished from income.
12. Finance . a capital sum, as distinguished from interest or profit.
13. Music .
a. an organ stop.
b. the subject of a fugue.
14. (in a framed structure) a member, as a truss, upon which adjacent or similar members depend for support or reinforcement.
15. each of the combatants in a duel, as distinguished from the seconds.
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Origin:
1250–1300; Middle English < Latin prīncipālis first, chief, equivalent to prīncip- ( see prince) + -ālis -al1

The noun principle and the noun and adjective principal are often confused. Although pronounced alike, the words are not interchangeable in writing. A principle is broadly “a rule of action or conduct” ( His overriding principle is greed ) or “a fundamental doctrine or tenet” ( Their principles do not permit the use of alcoholic beverages ). The adjective principal has the general sense “chief, first, foremost”: My principal objection is the cost of the project. The noun principal has among other meanings “the head or director of a school” ( The faculty supported the principal in her negotiations with the board ) and “a capital sum, as distinguished from interest or profit” ( The monthly payments go mostly for interest, leaving the principal practically untouched ).

Dictionary.com Unabridged
Based on the Random House Dictionary, © Random House, Inc. 2011.
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