Drop out   

Tape drop out is a brief but noticeable and undesireable loss or attenuation of the recorded signal.
Drop out can be caused by imperfections in the oxide layer of the tape or dust particles momentarily lifting the tape away from the record head. 
It is recommended that new tape be fast forward to the end and then rewound before use.  The notion here is that any loose particles on the tape will drop off during this exercise.  
Drop out can also be the result of careless handling of recording tape. Oil from fingers and hands can adhere to the surface of tape and attract dust and other contaminants resulting in a barrier between the oxide layer and the playback head.