(crosswords, often in combination) A word that runs horizontally in the completed puzzle grid or its associated clue.

"I solved all of the acrosses, but then got stuck on 3 down."

From one side to the other.

"she helped the blind man across;  the river is half a mile across"

On the other side.

"If we sail off at noon, when will we be across?"

In a particular direction.

"He leaned across for a book."

(crosswords) Horizontally.

"I got stuck on 4 across."

To, toward or from the far side of (something that lies between two points of interest).

"Fortunately, there was a bridge across the river."

On the opposite side of (something that lies between two points of interest).

"That store is across the street."

Across from: on the opposite side, relative to something that lies between, from (a point of interest).

From one side to the other within (a space being traversed).

"Could you slide that across the table to me, please?"

At or near the far end of (a space).

Spanning.

"This poetry speaks across the centuries."

Throughout.

"All across the country, voters were communicating their representatives."

So as to intersect or pass through or over at an angle.

"Lay the top stick across the bottom one."

In possession of full, up-to-date information about; abreast of.