Which expression refers to exactly halfway through the hour in Spanish?

The expression that means exactly halfway through the hour in Spanish is "y media." This phrase is commonly used when telling time to indicate that it is thirty minutes past the hour. For example, if it is 1:30, you would say "Es la una y media." This formulation applies universally, regardless of the hour, making it a fundamental part of telling time in Spanish.

In contrast, "a la una" translates to "at one o'clock," indicating a specific hour rather than the halfway point. "Y cuarto" means "a quarter past" the hour, which refers to fifteen minutes past the hour, not thirty. "A las dos" simply means "at two o'clock," which also does not convey the idea of being halfway through any hour. Therefore, "y media" is the only expression that accurately describes the concept of thirty minutes past the hour.