Lemons are in fact considered to be a citrus fruit because of the fact that they contain high amounts of citric acid. A fruit is categorized as being either acidic or alkaline based on the pH scale which measures acidity. If a fruit has a high acid content, it is acidic and if it has a very low acid content, it is considered to be alkaline.
Some examples of other acidic fruits include oranges, limes, and grapefruits. These are all fruits which are important for people who have acid reflux to avoid or limit in their diet because of the fact that they can make this particular problem significantly worse.