What is meant by Ems? (Android TextView)

Question

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 1

It is the width of the letter M in a given English font size.
So 2em is twice the width of the letter M in this given font.
For a non-English font, it is the width of the widest letter in that font. This width size in pixels is different than the width size of the M in the English font but it is still 1em.
So if I use a text with 12sp in an English font, 1em is relative to this 12sp English font; using an Italian font with 12sp gives 1em that is different in pixels width than the English one.