from one side to the other of (a place, area, etc.)

I ran across the street

she is travelling across the US on a promotional tour

expressing position or orientation in relation to (an area or passage)

they lived across the street from one another

the bridge across the river

from one side to the other of a place, area, etc.

he had swum across

used to express position or orientation

he looked across at me

halfway across, Jenny jumped

referring to a crossword answer which reads horizontally

19 across