"Spontaneous applause" means clapping or cheering that happens unexpectedly and without planning, usually due to something impressive or enjoyable happening. Full definition
The achievement was so impressive that the assembled experts broke into spontaneous applause (a rare occurrence at scientific conferences). (scientificamerican.com)
Both films are clearly products of their time, with numerous anecdotes relating spontaneous applause and standing curtain calls during their original theatrical runs. (filmfreakcentral.net)
When groups walked forward to perform, seated classmates whispered «buena suerte,» or good luck, erupting afterward in spontaneous applause, regardless of whether what they'd seen was any good. (educationnext.org)