Acids are generally liquids that, when dissolved in water, give off a solution of hydrogen ion activity greater than pure water. Another method to classify acids are if they donate protons. However, borane can not fit, and so the Lewis definition is the broadest and makes it accept electrons, opposed to its proton donor behaviour. There are many acids used for different purposes such as vinegar (which contains acetate) and lemon juice for flavor.
Acidic solutions have a pH of less than 7.