Siddha is the name of a group of demi-gods in the heaven.
.They are called so due to their uncanny powers ( or siddhis. ) . Many puranas mentioned Siddhas as follwers of Indra in his wars against the demons. In Mahaabhaaratham too, this race is mentioned many times.
The often mentioned demi-god groups are Rudras,Maruths, Aadithyas, Vasus, Siddhas, Saadhyas, Gandharvas, Garudas, Nagas, Kinnaras, Kimpurushas, Vidyaadharas, Yakshas or Guhyakas etc. Some groups have a fixed number of entities in them. For example, Rudras are eleven in number, Aadithyas are twelve, Maruths are seven and the like.