to rush in forcibly or violently
said esp of birds: to migrate into an area in abnormally large numbers
said of a natural population: to undergo a sudden upsurge in numbers, esp when natural ecological balances and checks are disturbed
irruption /-sh(ə)n/ noun
irruptive /-tiv/ adj
irruptively adv
[Latin irruptus, past part. of irrumpere to break in, from in-2 + rumpere to break]