“Miracles” in Islamic theological terms are acts that surpass normal limits that challenge others to do the same and are in accordance with the claims of the person performing them. What is meant by surpassing normal limits is that it seems contrary to the laws of nature.
Of course, when it is said that miracles surpass normal limits, not having a cause or negating the law of causality isn't intended; what is meant is that despite these phenomena having their own natural causes, these causes are unknown and inaccessible to the people. What is meant by ‘challenge’ is that the prophet performing the miracle challenges those rejecting his call and his miracle to do something similar. Miracles are done by the prophets themselves and with the permission of Allah, meaning that the miracle originates from the never ending power of Allah (swt), thus always making it prevalent. When it comes to miracles, there is no need of learning or practice, and as a result, it is unconditional and can take place no matter what. Prophets’ miracles aren't done out of amusement, etc., but are done with the intention of guiding humanity; this is what sets miracles apart from other uncommon phenomena such as the answering of prayers, etc., and magic and sorcery.
In such phenomena, there is no sign of challenges or guidance or claims of prophethood.
Our research and search yielded nothing regarding the imams denying themselves of supernatural powers; on the contrary, there are innumerous narrations of their miracles and supernatural acts, to the extent that in the book of Bihar al-Anwar, under the name of each imam, there is a chapter on that imam’s miracles, narrating all the hadiths there are in that regard.
If you ever come across a hadith that negates the supernatural powers of the imams and their ability to intervene in worldly phenomena, please inform us so that we can do necessary research on it.
For further information, see: The Definition and Verification of Miracles, Question 115 (website: 985).
 With help from Question 115 (website: 985).