Bhagavad Gita: Chapter 15, Verse 16

द्वाविमौ पुरुषौ लोके क्षरश्चाक्षर एव च |
क्षर: सर्वाणि भूतानि कूटस्थोऽक्षर उच्यते || 16||

dvāv imau puruṣhau loke kṣharaśh chākṣhara eva cha
kṣharaḥ sarvāṇi bhūtāni kūṭa-stho ’kṣhara uchyate

dvautwo; imauthese; puruṣhaubeings; lokein creation; kṣharaḥthe perishable; chaand; akṣharaḥthe imperishable; evaeven; chaand; kṣharaḥthe perishable; sarvāṇiall; bhūtānibeings; kūṭa-sthaḥthe liberated; akṣharaḥthe imperishable; uchyateis said


BG 15.16: There are two kinds of beings in creation, the kṣhar (perishable) and the akṣhar (imperishable). The perishable are all beings in the material realm. The imperishable are the the liberated beings.


In the material realm, Maya binds the individual soul to the material body.  Although the soul itself is eternal, it repeatedly experiences the phenomenon of birth and death of the body.  Thus, Shree Krishna calls the embodied living entities in the material world as kṣhar (perishable).  This includes all the beings from the tiniest insect to the highest celestial gods.  

Apart from these are the souls in the divine realm, the Abode of God.  These souls possess an immortal body in which they do not have to experience the phenomenon of death, and hence they are categorized as akṣhar (imperishable).