Achala Murti in Hinduism is the term used to refer to the fixed and immovable murti (idol or icon) in a Hindu temple. It is the main murti in a temple and it is installed on a pedestal at the center of the sanctum sanctorum.
Achala Murti is usually made of granite. In the case of Shiva, it is a Shivling. In the case of Goddess Kali, the immovable icon may be made of jackwood. Some Vishnu murtis are made using Salagrama.
A special kind of mortar called kastusarkarayoga is sometimes used to make the achala murti. It may be in the sitting position, standing position or inclined position.
This kind of immovable murti is also called Dhruva Bera.
Chalamurtis (portable smaller images) of the deity are used in various public routines of worship, and bathing rites and are called utsava murti or utsavabera when used for a festival procession.
During the Shri Bahubali offerings made to the various minor deities by the priest (tantri, pujai or archaka) under the supervision of the deity, it is the chalamurti that is carried by the assistant priest. During such processions, the god’s presence is transferred into the smaller movable murti.
If the main Achala Murti is in a standing position, the small chalamurti must also be standing. If the main immovable murti is sitting, the portable image may be in the sitting posture or in the standing position. If the fixed main murti is in an inclined position, the moveable murti must be either standing or sitting.
After the procession, the proper way of keeping the movable image near the main deity has also been prescribed. In the Jagannath temple at Puri, the three achalamurti are made of wood and are renewed periodically.