Wikipedia
Hiraṇyagarbha ( Sanskrit: हिरण्यगर्भः ; literally the 'golden womb' or 'golden egg', poetically rendered in translation as 'universal germ') is the source of the creation of [universe]] or the manifested cosmos in Hindu philosophy. It finds mention in one hymn of the Ṛgveda ( RV 10.121), known as the Hiraṇyagarbha Sūkta, suggesting a single creator deity (verse 8: , Griffith: "He is the God of gods, and none beside him."), identified in the hymn as Prajāpati. The concept of the "golden womb" is again mentioned in the Vishvakarman Sūkta (RV 10.82).
The Upanishad calls it the Soul of the Universe or Brahman, and elaborates that Hiraṇyagarbha floated around in emptiness and the darkness of the non-existence for about a year, and then broke into two halves which formed the Svarga and the Pṛthvi.
In classical Purāṇic Hinduism, Hiraṇyagarbha is the term used in the Vedanta for the "creator". Hiraṇyagarbha is also Brahmā, so called because he was born from a golden egg ( Manu Smṛti 1.9), while the Mahābhārata calls it the Manifest.
Hiraṇyagarbha is also a sacrifice, where it is known as Hiraṇyagarbha Yagna. This sacrifice was performed by the founder of the Rashtrakuta dynasty, Dantidurga, to confer Kshatriyahood on himself, as he was not born a Kshatriya, and in olden days, the Caste System in India was very prevalent and he was not accepted by the people as their King, until he conferred Kshatriyahood upon himself.
Some classical yoga traditions consider Hiraṇyagarbha as the originator of yoga, though this may also be a name for Rishi Kapila.