This 2003 Toyota 4Runner battery post is a perfect example of why you should look under your hood and inspect things on a regular basis. It does not take long for a battery terminal to get corroded. Also with corrosion this bad something else is going on. For some reason there is battery acid leaking near this battery post. A good clean up is needed. Of course you can use baking soda and water as we have all been told to clean this up. I use a purple degreaser in a spray bottle and it works quite well. You have to use a base or alkaline solution to neutralize and acid. The standard purple degreaser is an alkaline solution. Remember though that in the process of neutralizing an acid with a base that a salt is formed and metals do not like salts. Be sure to rinse thoroughly.
A good soaking with the alkaline degreaser along with a thorough rinsing and the terminal is quite clean. You can also see the reason for the corrosion. See the splits in the plastic case around the battery post.
Luckily there was no damage to the battery cable terminal end. A new battery and a little more cleaning and this one was done, except for the a/c repair I still have to do.