Formal:
* Interlude: A brief period of time when an activity or event is stopped.
* Digression: A temporary departure from the main subject.
* Pause: A temporary stop or break.
* Breach: A temporary interruption of a continuous process.
Informal:
* Break: A short interruption.
* Stop: A brief cessation of activity.
* HICCUP: A brief interruption or setback.
* LAG: A temporary delay or interruption in a process.
Specific to conversation:
* Interjection: A short exclamation, often interrupting a conversation.
* Aside: A brief remark or comment made by a character in a play to the audience.
The best word to use will depend on the specific situation.