An outcry is an outburst or shout — either by one person, or by a group of people. Faced with the outcry of three kids under the age of ten, a frustrated babysitter may give in, letting them stay up past bedtime.
When a teacher promises his class a pizza party at the end of the week, he can expect an outcry if the pizzas don't show up on Friday. This kind of outcry is a form of protest, a group of angry or passionate people raising their voices together. The older, fourteenth century meaning of outcry is simply "crying out." The "public protest" meaning was first used around 1910.