In terms of the name of a medical condition, here's what Wikipedia says about pseudobulbar affect:
Pseudobulbar affect (PBA), or emotional incontinence, is a type of emotional disturbance characterized by uncontrollable episodes of crying and/or laughing, or other emotional displays. PBA occurs secondary to a neurologic disorder or brain injury. Patients may find themselves crying uncontrollably at something that is only moderately sad, being unable to stop themselves for several minutes. Episodes may also be mood-incongruent: a patient may laugh uncontrollably when angry or frustrated, for example. Sometimes, the episodes may switch between emotional states, resulting in the patient crying uncontrollably before dissolving into fits of laughter.
The pseudobulbar affect, also referred to as emotional lability, should not be confused with labile mood or labile emotions that stem from emotional instability – affective dysregulation – commonly seen in personality disorders, such as borderline personality disorder.
Note that it's not specific only to laughter.