A word that runs horizontally in the completed puzzle grid or its associated clue.
"The crossword solver struggled to find an answer across before moving on down."
"The river is narrow enough to swim across."
From one side to the other.
"She swam across the lake to reach the island on the other shore."
In plain English: Across means from one side to the other of something, like walking across the street.
"We walked across to get some ice cream from the other side of the street."
To, toward, or from the far side of (something that lies between two points of interest).
"She reached across the table to shake his hand."
The word "across" comes from Old French and originally meant being in or upon something shaped like a cross. It entered English through Anglo-Norman phrases that combined elements meaning "in" with the shape of a cross.