day trade

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

• noun A trade of a security that is opened and closed on the same day.

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

• noun business, finance A transaction in which units of stock or other investment instruments are purchased and then sold within a single market day.
• verb business, finance To engage, as a deliberate investment strategy, in the very short-term trading of stocks or other securities, by selling any given security within the same market day during which it was purchased.