Entertaiment is the act of entertaining. It can also refer to:
Something affording pleasure, diversion, or amusement: a pleasant day out on the lake; solving a crossword puzzle; a theatrical performance.
Entertainment has become a highly diverse field and can be adapted to any scale: from an intimate activity that stimulates the brain to realease seratonin, dopamine and other feel good chemicals, to events for thousands. It can have a serious purpose, as in the case of ceremony, religious festival or satire for example.
In some industries it is common to abbreviate the word Entertaiment to entmt. You might see this on a flier or in industry news publications where space is limited. However, it is not commonly used in general prose. More often you will find the word entertaiment spelled out in full, as it should be.