Premise

A premise or premiss is a statement that an argument claims will induce or justify a conclusion. It is an assumption that something is true. 
Explanation.
In logic, an argument requires a set of (at least) two declarative sentences (or "propositions") known as the "premises" (or "premisses"), along with another declarative sentence (or "proposition"), known as the conclusion. This structure of two premises and one conclusion forms the basic argumentative structure. More complex arguments can use a sequence of rules to connect several premises to one conclusion, or to derive a number of conclusions from the original premises which then act as premises for additional conclusions. An example of this is the use of the rules of inference found within symbolic logic.
Aristotle held that any logical argument could be reduced to two premises and a conclusion. Premises are sometimes left unstated, in which case, they are called missing premises, for example:
Socrates is mortal because all men are mortal.
It is evident that a tacitly understood claim is that Socrates is a man. The fully expressed reasoning is thus:
Because all men are mortal and Socrates is a man, Socrates is mortal.
In this example, the independent clauses preceding the comma (namely, "all men are mortal" and "Socrates is a man") are the premises, while "Socrates is mortal" is the conclusion.
The proof of a conclusion depends on both the truth of the premises and the validity of the argument. Also, additional information is required over and above the meaning of the premise to determine if the full meaning of the conclusion coincides with what is.
For Euclid, premises constitute two of the three propositions in a syllogism, with the other being the conclusion. These categorical propositions contain three terms: subject and predicate of the conclusion, and the middle term. The subject of the conclusion is called the minor term while the predicate is the major term. The premise that contains the middle term and major term is called the major premise while the premise that contains the middle term and minor term is called the minor premise.
A premise can also be an indicator word if statements have been combined into a logical argument and such word functions to mark the role of one or more of the statements. It indicates that the statement it is attached to is a premise.