What is the difference between "appropriate" and "suitable"?

What is the difference between "appropriate" and "suitable"?

In many contexts they are exactly equivalent. However suitable has the meaning of simply being usable for a particular purpose, while appropriate has additional connotations of being non-offensive, of the proper register, etc.

The definitions shown by the NOAD are the following:

• suitable: right or appropriate for a particular person, purpose, or situation
• appropriate: suitable or proper in the circumstances

The difference is that you say "these toys are not suitable for children under five" (appropriate for a particular person), but "a measure appropriate to a wartime economy" (proper in the circumstances). You can also say "this watch is suitable for scuba diving".

Let's take kiamlaluno's example:

This watch is suitable for scuba diving.

Let's say for example that the watch was given to you by your friend's recently-deceased father, and you are going diving with your friend. While the watch suits the purpose of scuba diving, it may not be appropriate to wear it while with this friend, as it reminds him of his father.