  The Onsager Lie algebra $O$ is often used to study integrable lattice models.
The universal enveloping algebra of $O$ admits a $q$-deformation $O_q$ called
the $q$-Onsager algebra. Recently, an algebra $\mathcal O_q$ was introduced
called the alternating central extension of $O_q$. In this paper we introduce a
Lie algebra $\mathcal O$ that is roughly described by the following two
analogies: (i) $\mathcal O$ is to $O$ as $\mathcal O_q$ is to $O_q$; (ii) $O_q$
is to $O$ as $\mathcal O_q$ is to $\mathcal O$. We call $\mathcal O$ the
alternating central extension of $O$. This paper contains a comprehensive
description of $\mathcal O$.
