What is one part of the flag referred to as?

The correct term for one part of the flag is the "Union." In the context of flags, especially in the United States flag, the "Union" refers specifically to the blue canton or field that contains the stars, symbolizing the union of the states. The Union is a significant aspect of the flag's design, representing the collective unity of the states within the nation.

The other terms do not accurately represent parts of a flag. The "blue field" is a descriptive term, but it does not capture the specific terminology used in flag nomenclature. "Spade" is not a standard term used to describe any element of a flag and may lead to confusion as it generally refers to a shape rather than a flag component. Finally, "grommet" refers to a reinforcing ring or hole typically found in various types of fabric or banners for hanging or displaying, but it does not designate a part of the flag itself.