base (bās)

1. The part of an organ nearest its point of attachment.
2. A fundamental ingredient; a chief constituent of a mixture.
3. Any of a large class of compounds, including the hydroxides and oxides of metals, having a bitter taste, a slippery solution, the capacity to turn litmus blue, and to react with acids to form salts.
4. A molecular or ionic substance capable of combining with a proton to form a new substance. Also called Brønsted base.
5. A nitrogen-containing organic compound that combines in such a manner.
6. A substance that provides a pair of electrons for a covalent bond with an acid.