How acidic or basic a solution is. 7 is neutral (water), less than 7 is acidic, and more than 7 is basic.

Values below about 2 or above 11.5 are considered corrosive by OHSA but that's only a small part of the picture. You could have a solution of arsenic and dog poop that was pH 7 and you still wouldn't want to get near it.

In "pH" the "H" stands for "Hydrogen" and the "p" stands for "negative one times the common logarithm of the concentration of." Don't ask.

Image CC BY 3.0

https://cnx.org/contents/FPtK1zmh@8.108:5ORVCb_A@4/Inorganic-Compounds-Essential-Illustration

from Anatomy & Physiology, Connexions Web site.

http://cnx.org/content/col11496/1.6/, Jun 19, 2013.