What is a high-temperature physical state of matter in which atoms lose their electrons?

A plasma.  

Step-by-step explanation:

A plasma is a hot, ionized gas in which the atoms have lost one or more of their electrons.

Thus, the plasma consists gaseous ions as well as the electrons that were stripped from them.

The Sun's high temperatures strip the electrons from its hydrogen and helium atoms, so the Sun is essentially a giant ball of plasma.