To laugh repeatedly in a quiet but uncontrolled way, often something silly or rude or when you are nervous. (Cambridge Dictionary)
To laugh in a silly, often high-pitched way, especially with short, repeated gasps and titters, as from juvenile or ill-concealed amusement or nervous embarrassment. (Dictionary.com)
Laugh lightly and repeatedly in a silly way, from amusement, nervousness, or embarrassment. (Oxford Dictionaries)
To laugh with repeated short catches of the breath. (Merriam – Webster)
If someone giggles, they laugh in a childlike way, because they are amused, nervous, or embarrassed. (Collins Dictionary)
Why we get the giggles: Scientist reveals how social bonds can cause contagious laughter – even when something isn’t funny.