BrE /dɪˈkriːs/ ; NAmE /dɪˈkriːs/ [intransitive, transitive] (rather formal) to become or make something become smaller in size, number, etc. decrease (from something) (to something) The number of new students decreased from 210 to 160 this year. a decreasing population Donations have decreased significantly over the past few years. decrease by something The price of wheat has decreased by 5%. decrease in something This species of bird is decreasing in numbers every year. decrease something People should decrease the amount of fat they eat.