under [(1) below; (2) below the surface of; (3) less than; (4) as called for by a law, agreement or system]  

The river flows under the bridge. (1)

 

The submarine went under. (2)

 

The book was under 20 dollars. (3)

 

Such action is not permitted under the law. (4)

 

understand [(1) to know what is meant; (2) to have knowledge of]  

I understand what you mean. (1)

 

He understands chemistry. (2)

 

unite [to join together]  

The new president will try to unite all parties in support of his program.

 

universe [all of space, including planets and stars]  

Do you know why the universe is expanding?

 

university [a place of education that usually includes several colleges and research organizations]  

Her sons are students at different universities.

 

unless [(1) except if it happens; (2) on condition that]  

I will not go, unless the rain stops. (1)

 

I will come to your house unless it rains. (2)

 

until [(1) up to a time; (2) before]  

She said she would wait for me until three o'clock. (1)

 

We cannot start until everyone arrives. (2)