What's the difference between a nucleophile and a base?
The two are related, in that most nucleophiles are (Lewis) bases and vice versa. Some good nucleophiles are also strong bases, e.g. $\ce{HO-}$. However, a species can be a good nucleophile and a weak base, e.g. $\ce{I-}$; or a species can be a weak nucleophile and a strong base, e.g. $\ce{t-BuO-}$. How can we separate this behavior?


  Nucleophilicity is a kinetic phenomenon.


Nucleophilicity is most often defined based on the relative rate of the reactions of nucleophiles with a standard substrate in a standard solvent.

For example, a standard reaction might look like:

$$\ce{CH3I -&gt;[Nu-][H2O] CH3Nu}$$

The nucleophilicity will be related to the relative rate constant of reaction with the nucleophile (relative to the rate constant of the reaction with water $\equiv 1$).


  Basicity is a thermodynamic phenomenon.


Basicity is based on the position of equilibrium:

$$\ce{B + HSol &lt;=&gt; BH+ + Sol-}$$