The difference between Constant and Variable

When used as nouns, constant means that which is permanent or invariable, whereas variable means something that is variable.

When used as adjectives, constant means unchanged through time or space, whereas variable means able to vary.

check bellow for the other definitions of Constant and Variable

Constant as an adjective:

Unchanged through time or space; permanent.
Constant as an adjective:

Consistently recurring over time; persistent.
Constant as an adjective:

Steady in purpose, action, feeling, etc.
Constant as an adjective:

Firm; solid; not fluid.
Constant as an adjective (obsolete):

Consistent; logical.

Constant as a noun:

That which is permanent or invariable.
Constant as a noun (algebra):

A quantity that remains at a fixed value throughout a given discussion.
Constant as a noun (science):

Any property of an experiment, determined numerically, that does not change under given circumstances.
Constant as a noun (computing):

An identifier that is bound to an invariant value; a fixed value given a name to aid in readability of source code.

Variable as an adjective:

Able to vary.

Examples:

"variable winds or seasons; a variable quantity"
Variable as an adjective:

Likely to vary.
Variable as an adjective:

Marked by diversity or difference.
Variable as an adjective (mathematics):

Having no fixed quantitative value.
Variable as an adjective (biology):

Tending to deviate from a normal or recognized type.

Variable as a noun:

Something that is variable.
Variable as a noun:

Something whose value may be dictated or discovered.

Examples:

"There are several variables to consider here."
Variable as a noun (mathematics):

A quantity that may assume any one of a set of values.
Variable as a noun (mathematics):

A symbol representing a variable.
Variable as a noun (programming):

A named memory location in which a program can store intermediate results and from which it can read them.
Variable as a noun (astronomy):

A variable star.
Variable as a noun (nautical):

A shifting wind, or one that varies in force.
Variable as a noun (nautical, in the plural):

Those parts of the sea where a steady wind is not expected, especially the parts between the trade-wind belts.