drop letter

from The American Heritage® Dictionary of the English Language, 5th Edition.

• noun A letter that is mailed and delivered from the same post office.

from Wiktionary, Creative Commons Attribution/Share-Alike License.

• noun US A letter mailed at a post office and either kept for general delivery, or delivered to a post office box, at that same post office.
• noun typography A large letter at the beginning of a line of text, positioned in a decorative way below the baseline of the rest of the type.