afterthought

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

• noun An idea, response, or explanation that occurs to one after an event or decision.

from The Century Dictionary.

• noun A later or second thought.
• noun Reflection after an act; some consideration that occurs to one's mind too late, or after the performance of the act to which it refers.

from the GNU version of the Collaborative International Dictionary of English.

• noun Reflection after an act; later or subsequent thought.
• noun an action taken after another action and related to the first action, which would normally or optimally be done along with the first action; -- .
• noun a feature or part added to a device, not thought of in its original design.

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

• noun A reflection after an act; a later or subsequent thought, action, or expedient.
• noun Something additional to the original plan or concept.
• verb To expound as an afterthought.

from WordNet 3.0 Copyright 2006 by Princeton University. All rights reserved.

• noun an addition that was not included in the original plan
• noun thinking again about a choice previously made