Ship's emergency transmitter (also: ship's emergency radio transmitter) is – according to article 1.99 of the International Telecommunication Union´s (ITU) ITU Radio Regulations (RR) – defined as «A ship's transmitter to be used exclusively on a distress frequency for distress, urgency or safety purposes.»
Each transmitter shall be classified by the service in which it operates permanently or temporarily.
References
Ship's emergency transmitter Wikipedia(Text) CC BY-SA