下半天

afternoon

down / downwards / below / lower / later / next (week etc) / second (of two parts) / to decline / to go down / to arrive at (a decision, conclusion etc) / measure word to show the frequency of an action

second half

half / semi- / incomplete / (after a number) and a half

half of the day / a long time / quite a while / midair / CL:個|个[ge4]

day / sky / heaven