Addition is when you put two or more numbers or items together thus resulting in a greater whole.
2+5=7
2+5+1=8
I'm not going to explain adding negative numbers. :-)

Addition can also be used as an expression. To do something in addition to something else.

Addition is also one of the rules of propositional logic. And it's a strange one.

P
Therefore, P∨Q (P or Q)

I have an apple, therefore I have an apple or an orange. Obviously true, but not generally useful. Note that having this rule means that if you have managed to show both P and ¬P (not P), you can proove anything.

P
P OR I am God (by addition)
¬P
Therefore, I am God.

This is also called disjunction introduction, although one cannot introduce the disjunction 'and' in the same way one can introduce 'or'.

Back up to Rules of Inference

1.

The act of adding two or more things together; -- opposed to subtraction or diminution.

Locke.

2.

Anything added; increase; augmentation; as, a piazza is an addition to a building.

3. Math.

That part of arithmetic which treats of adding numbers.

4. Mus.

A dot at the right side of a note as an indication that its sound is to be lengthened one half.

[R.]

5. Law

A title annexed to a man's name, to identify him more precisely; as, John Doe, Esq.; Richard Roe, Gent.; Robert Dale, Mason; Thomas Way, of New York; a mark of distinction; a title.

6. Her.

Something added to a coat of arms, as a mark of honor; -- opposed to abatement.

Vector addition Geom., that kind of addition of two lines, or vectors, AB and BC, by which their sum is regarded as the line, or vector, AC.

Syn. -- Increase; accession; augmentation; appendage; adjunct.

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