union jack

Short definition from Wiktionary

Noun

1. 1
   Alternative letter-case form of Union Jack
2. 2
   The flag of the United Kingdom, consisting of the flags of England (St. George's Cross), Scotland (St. Andrew's Cross), and Ireland (St. Patrick's Cross, now only used in Northern Ireland) combined.
3. 3
   The British Empire.
4. 4
   The flag, consisting of the flags of England, Scotland, and Northern Ireland combined, flown on ships of the government of the United Kingdom.
5. 5
   The flag, consisting of the blue star-studded field in the corner of the national ensign, flown at the jackstaff by ships at anchor.