Afterthought (noun)

1. A thought that occurs later, often as a realization or correction.
2. A consideration or action that is added later, often as an after effect.

Origin:

From after ("following") + thought ("a mental process").

Examples:

1. The dessert was an afterthought, added to the menu at the last minute.
2. He realized, as an afterthought, that he had forgotten to lock the door.
3. He made the comment as an afterthought, not thinking it would cause offense.
4. Safety was an afterthought during the construction of the building.
5. She had an afterthought, and decided to bring an umbrella before leaving the house.