nouns
a sudden violent entrance; a bursting in
"the recent irruption of bad manners"
is a type of (narrower)
the act of entering
"she made a grand entrance"
a sudden sharp increase in the relative numbers of a population
is a type of (narrower)
increase in the number of people who inhabit a territory or state
a sudden violent spontaneous occurrence (usually of some undesirable condition)
"the outbreak of hostilities"