The Lewis definition is a substance that can donate a pair of electrons. This does not require a proton or aqueous solution. There are several other definitions as well. An basic solution is defined as one that has a pH greater than 7.0. Basic materials or solutions are sometimes called caustic or alkaline .
Water can also act as a weak acid or base.
Just because an base is weak does not mean that it can't harm you. For example, ammonia can cause severe burning of the lungs and death if enough is inhaled and is extremely irritating to the eyes. It is important to know the pH of substances because they may be corrosive or react with incompatible materials. For example, acids and bases should not be stored or used near each other as their accidental combination could generate a huge amount of heat and energy, possibly resulting in an explosion. Whenever a substance enters the eye, flush with water for 15 minutes and get prompt medical attention.