SB 5.20.44

mṛte ’ṇḍa eṣa etasmin yad abhūt tato mārtaṇḍa iti vyapadeśaḥ; hiraṇyagarbha iti yad dhiraṇyāṇḍa-samudbhavaḥ.
Synonyms: 
mṛte — dead; aṇḍe — in the globe; eṣaḥ — this; etasmin — in this; yat — which; abhūt — entered personally at the time of creation; tataḥ — from that; mārtaṇḍa — Mārtaṇḍa; iti — thus; vyapadeśaḥ — the designation; hiraṇya-garbhaḥ — known as Hiraṇyagarbha; iti — thus; yat — because; hiraṇya-aṇḍa-samudbhavaḥ — his material body was created from Hiraṇyagarbha.
Translation: 
The sun-god is also known as Vairāja, the total material body for all living entities. Because he entered this dull egg of the universe at the time of creation, he is also called Mārtaṇḍa. He is also known as Hiraṇyagarbha because he received his material body from Hiraṇyagarbha [Lord Brahmā].
Purport: 

The post of Lord Brahmā is meant for very highly elevated, spiritually advanced living beings. When such living beings are unavailable, Lord Viṣṇu, the Supreme Personality of Godhead, expands Himself as Lord Brahmā. This takes place very rarely. Consequently there are two kinds of Brahmās. Sometimes Brahmā is an ordinary living entity, and at other times Brahmā is the Supreme Personality of Godhead. The Brahmā spoken of here is an ordinary living being. Whether Brahmā is the Supreme Personality of Godhead or an ordinary living being, he is known as Vairāja Brahmā and Hiraṇyagarbha Brahmā. Therefore the sun-god is also accepted as Vairāja Brahmā.