ojačati

1. to strengthen; to reinforce
   Treba da ojačaš mišiće leđa. — You need to strengthen your back muscles.
   Vlada želi da ojača domaću privredu. — The government wants to strengthen the domestic economy.
2. (without an object) to grow stronger; to get stronger
   Posle bolesti je brzo ojačao. — He quickly got his strength back after the illness.
   Vetar je ojačao tokom noći. — The wind grew stronger during the night.

Used both with an object (ojačati mišiće) and without one (vetar je ojačao) meaning "to grow stronger".