What is the difference between a Postulate and a Definition?

Hello.

"What is a Postulate?
 * A POSTULATE is a statement that is assumed to be true.
 * A POSTULATE is an initial proposition or statement that is accepted
as true without proof and from which further statements, or theorems,
can be derived."
source:
e-zgeometry.com
http://www.e-zgeometry.com/classph/sec1/1.2/1.2.htm

" A postulate is basically something that you assume is true..."
source: physics.weber.edu
http://physics.weber.edu/arnold/courses/2003fall/PHSX%201030/Lectures/lecture%2024.htm

"a definition merely names an object..."
source:
Math Dictionary
http://www.mathsyear2000.org/dictionary/g_nojscript.html


"A postulate is a statement that might be questioned or denied without
absurdity. A definition, or rather, one of the pair, or larger number,
of propositions that constitute a definition, cannot be questioned,
because it merely states the logical relation of a conception thereby
introduced to conceptions already in use."
source:
The Collected Works of Charles Sanders Peirce (325)
cited in the PEIRCE mailing list:
http://members.door.net/arisbe/menu/people/peirce-l/1297.htm


An example of a postulate: "There is a unique straight line between
any two points."

Our experience tells us that this seems to be true, so we assume it to
be true without proof.

An example of a definition: "A triangle has three sides."  

See the difference?  That a triangle has three sides isn't something
that we assume but can't prove.  There's no issue of "proving" or
"disproving" a definition.  We call something that has three sides a
"triangle." Proof isn't even an issue.

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search strategy:
"a postulate is", "a definition is"
"postulate", statement, true, proof, assume

I hope this helps.

thank you for your help. You explained it so that it is understandable.