In chemistry, pH is a measure of how acidic something is. The lower the number on the pH scale, the more acid a substance contains.
The pH scale goes from 0 to 14, with 0 being the most acidic. The higher, low-acid numbers on the pH scale are basic — they feel slippery and have an astringent taste. Substances at around 7, the middle of the pH scale, are neutral, like water. Lemon juice has a pH of 2.2, since it's acidic. pH comes from the German word Potenz, "potency or power," and H, the chemical symbol for hydrogen.