Yajna (yagya or yajnya) is the process of creation. It is a process in which different elements come together, intermingle and transform into something new. It is a seamless, ceaseless process within us, and outside, enveloping all the elements of visible, invisible universes. In the Vedic texts, it is also referred to as Prajapati, Vishnu and Indra.
Contrary to popular meanings attributed to the term ( ``worship, devotion, prayer, praise, offering, sacrifice, oblation``), yajna in essence involves the interaction of two substances—one which is primary called vrisha and the secondary one referred to as yosha. In this interaction, the primary substance, also called annada, consumes the secondary one, often referred to as anna. It would be, however, erroneous to limit the meaning of anna to food or grain and annada to the one who consumes. Both these substances, in the Vedic sciences, are references to a phenomenon; it also denotes male and female principles whose union leads to creation.
Yajna could be easily understood by examining the process that takes place within our body. The body extracts juices from the food we eat and these juices, or rasa, give us energy. This energy is prana and stimulates all our activities. Once this energy dissipates, our body becomes weaker; this state of prana is called ashanaya which means `the desire to consume food`. Consumption of food produces juices that reinvigorate our body. This cycle continues incessantly and a cessation of this cycle means the end of prana or life.
In Bhagwad Gita, Krishna tells Arjuna that "Having first created all individuals by yajna, Prajapati said: 'By this shall you propagate, let this be your kamadhenu (that which fulfils all your aspirations).'"
Thus it is clear that yajna is the process of ``fusion, interaction and interface of the supraphysical energies which facilitate creation.`` Therefore, yajna performed by humans are not `rituals` but a homage to the process operating in the cosmic domain.
The first cosmic yajna was conceived to have been performed by Prajapati himself in which he created everything out of himself. The creation took place at two levels—one at the level of thought (Veda) and another at the material level which brought into existence all the visible worlds, consisting of creatures or living beings composed of three principles of mind, life and matter. Thus the yajna ritual became the first act of the devas or representatives of Prajapati and for those men who followed the tradition on earth.
 Rishi Kumar Mishra, The Cosmic Matrix, Rupa, 2001
 A S Ramanathan, The Yajna Concept of the Aryans, Indian Journal of History of Science, 21 (1); 7-14 (1986). Accessed at http://www.insa.nic.in/writereaddata/UpLoadedFiles/IJHS/Vol21_1_2_ASRamanathan.pdf