What is "species" in the context of an electrochemical cell? My familiarity with the word "species" begins and ends with the classification of living things so I am utterly baffled as to what it means in the context of this excerpt from "Electrochemical cell", Wikipedia:

An electrochemical cell consists of two half-cells. Each half-cell consists of an electrode, and an electrolyte. The two half-cells may use the same electrolyte, or they may use different electrolytes. The chemical reactions in the cell may involve the electrolyte, the electrodes or an external substance (as in fuel cells which may use hydrogen gas as a reactant). In a full electrochemical cell, species from one half-cell lose electrons (oxidation) to their electrode while species from the other half-cell gain electrons (reduction) from their electrode.

 A: In electrochemistry, a species is just something distinct that we have to put into our equations to work out what's going on. For example water is a species, but so is $\text{H}^+$ and $\text{OH}^-$. Likewise $\text{NaCl}$ is a species, but in practise it ionises almost totally so the only species present would be $\text{Na}^+$ and $\text{Cl}^-$.
Generally a species is anything that appears on its own in the formula for calculating concentrations. For example for water we write:
$$ K_\text{water} ~ = ~ \frac{\left[\text{H}^+\right]\left[\text{OH}^-\right]}{\left[\text{H}_2\text{O}\right]} \,,$$
where $K_\text{water}$ is the equilibrium constant for dissociation of water, and all three items on the right hand side are treated as separate species (though we normally write the activity of water, $\left[\text{H}_2\text{O}\right]$, as the constant value $1$).
A: Species in this context means "chemical species", i.e. a type of molecule. $\mathrm{H_2O}$ is one example of a chemical species, $\mathrm{NaCl}$ is another.
