jačati

1. (without an object) to grow stronger; to intensify
   Vetar jača prema večeri. — The wind is getting stronger toward evening.
   Dinar ove nedelje jača u odnosu na evro. — The dinar is strengthening against the euro this week.
2. (+ acc.) to strengthen
   Plivanje jača mišiće leđa. — Swimming strengthens the back muscles.
   Vitamin C jača imunitet. — Vitamin C boosts the immune system.

The perfective is ojačati.