A Mother ship is a science fiction craft, usually capable of space travel. Typically it contains smaller space fighters and serves as a mobile base. Usually they are used by aliens.
In the popular film Independence Day, the invading aliens used a mother ship.
- "I'm going back to the mother ship." Jack Campbell, in The Family Man.
When used outside of a Science Fiction setting the word is spelt mother ship, being two distinct words. In such cases the word refers to a larger vessel (usually sea-going) that is home and launching point to smaller craft (such as aircraft). However, the words mother ship are rarely used when describing such vessels in English, instead specific names are assigned to the type of vessel in question.
i.e. An aircraft carrier is a mother ship, but is almost always referred to as a carrier or aircraft carrier.