1.   It is a process that increases the visibility of your site in SERP.

2.   It is process to increase the traffic of your site from search engine.

3.   It is a process of getting traffic to a "free", "organic", "Editorial", or "natural" is listed on search engines.

4.   It is a process to increase your business goal and sale.

5.   It is the process of improving the volume and quality of traffic to a web site from search engines via "natural" Search.

All statement have aim to target online visibility yet I like -

It is the process of improving the volume and quality of traffic to a web site from search engines via "natural" Search.

All statements are true but i like fifth point a lot.

I will go with the 3rd and 4th point.....  I personally think those are two points which covers complete main goals of the SEO....

These all are good but 3rd one is great.
I want to go with it.

Once you understand the concept of SEO you can write 'n' number of definitions.

I would love to go with option 5:- It is the process of improving the volume and quality of traffic to a web site from search engines via "natural" Search.

I agree with
"  It is a process to increase your business goal and sale."

hi
SEO is the combination of all, so its better been define combing all the statement.

Hi.
All the definitions are true but i go with fifth one.

It combines all the statements overall and i like to go with 5th point.

SEO is the process of increase ROI (Rate of Interest).
best one according to me 

Search engine optimization (SEO) is the process of improving a website to increase the volume and quality of traffic from search engines.

Once you understand the concept of SEO you can write 'n' number of definitions.