What is meant by Ems? (Android TextView)

What is meant by Ems (related to a TextView)? For example in

android:ems     setEms(int)

Makes the TextView be exactly this many ems wide.

Answer

android:ems or setEms(n) sets the width of a TextView to fit a text of n 'M' letters regardless of the actual text extension and text size. See wikipedia Em unit

but only when the layout_width is set to "wrap_content". Other layout_width values override the ems width setting.

Adding an android:textSize attribute determines the physical width of the view to the textSize * length of a text of n 'M's set above.