Siddhi is a term from 📝Sanskrit that in 📝yoga names a miraculous power described as imparted by the late stages of intense 📝meditation, and in 📝Buddhism any occult power acquired through discipline.
Those two senses come from the English entry in the Random House Unabridged Dictionary, as published by Dictionary.com, which gives the word's origin as Sanskrit and lists the Pali form iddhi.
The Sanskrit word carries many more senses. The Monier-Williams Sanskrit-English dictionary lists accomplishment, performance, fulfilment, complete attainment of any object, and success; proof and indisputable conclusion; prosperity, personal success and good luck; and supreme felicity, bliss, beatitude, final emancipation and perfection. Among its many further senses is the acquisition of supernatural powers by magical means, or the faculty so acquired, of which the dictionary says eight are usually enumerated. The same entry records Siddhi as Success or Perfection personified, and as a name of Durgā.
In Pali, the Pali Text Society's Pali-English Dictionary defines siddhi as accomplishment, success and prosperity, and traces it to the root sidh.