A substance that can transfer ions from a hydrophilic medium such as water into a hydrophobic medium, such as hexane or a biological membrane, where the ions typically would not be soluble, also known as an ion carrier. The ions transferred are usually metal ions (for example, lithium (Li⁺), sodium (Na⁺), potassium (K⁺), magnesium (Mg²⁺), or calcium (Ca²⁺)); but there are ionophores that promote the transfer of other ions, such as ammonium ion (NH₄⁺) or amines of biological interest.