Siddhis is referred to spiritual power attained through meditation. Siddhis is derived from the Sanskrit word "Siddhi" meaning "perfection", "achievement", "execution" or "successes". Thus in both Hinduism and Buddhism, Siddhis mean perfection. Siddhis are mentioned with comparison to "Riddhi" or "Riddhis", which means material or worldly wealth, power and luxurious lifestyles. The spiritual powers attained through Siddhis vary from simple forms of clairvoyance to being able to increase the power. The spiritual power has the ability to be present at various places at one time, to become as small as an atom, to materialize objects and to remember the memories from past lives.

Achieving the Power of Siddhi
There are many perception of attaining Siddhis. In one case Siddhis are a normal set of occurrences that should not be focused upon because they will pull one from the path. In another perspective it is said that each Siddhi should be followed because it will allow one to understand the power of the God. Siddhis may occur naturally through the action or Karma, or by Sadhana. Siddhi is also attained through penance or austerities, that is Tapasya or it can be attained by grace.

Forms of Siddhis in Puranas
As mentioned in the Puranas there are Nine forms of Siddhis. They are mentioned below:

1. Parkaya Pravesha: Parkaya Pravesha means one"s soul entering into the body of some other person. In the course of knowledge even a deceased body can be brought to existence.
2. Haadi Vidya: The existence of Haadi Vidya is referred in many ancient texts. Once this knowledge is attained a person feels neither hungry nor thirsty, and can remain without eating food or drinking water for several days.
3. Kaadi Vidya: In Kaadi Vidya a person is not influenced by the change of seasons. After achieving the knowledge of Kaadi Vidya a person will not be affected with the change of season. He will not feel hot in summer, cold in winter and so on. Even he will not feel the heat if he sits in the fire.
4. Vayu Gaman Siddhi: With the help of Vayu Gaman Siddhi, a person can become capable of flying in the skies and travelling from one place to another in just a few seconds.
5. Madalasa Vidya: After attaining Madalasa Vidya a person becomes powerful of increasing or decreasing the size of his body according to his wish. Lord Hanuman had miniaturized his body through this Vidya while entering the city of Lanka.
6. Kanakdhara Siddhi: Kanakdhara Siddhi is the knowledge through which one can acquire immense and unlimited wealth.
7. Prakya Sadhana: Through Prakya Sadhana a person has the power to direct his disciple to take birth from the womb of a woman who is childless.
8. Surya Vigyan: The Surya Vigyan which is the solar science and is one of the most significant sciences of ancient India. With the help of this power one can be transformed into another form through the medium of sun rays.
9. Mrit Sanjeevani Vidya: Mrit Sanjeevani Vidya is the knowledge or Siddhi created by Guru Shukracharya. With the help of this Siddhi even a dead person can be brought back to life.

Forms of Siddhis in Mahabharata
In the epic Mahabharata it is mentioned that there are Eight Primary Siddhis or the "Ashta Siddhis". They are:

1. Anima means reducing one`s body even to the size of an atom.
2. Mahima Siddhi means expanding one`s body to an infinitely large size.
3. Garima means becoming infinitely heavy.
4. Laghima means becoming almost weightless.
5. Prapti signifies having unrestricted access to all places.
6. Prakamya is the Siddhi which helps in realizing whatever one desires.
7. Istva means possessing absolute lordship.
8. Vastva which refers to the supremacy to subjugate all.

In Hinduism, it is believed that Lord Hanuman possesses the ability to bestow the eight Siddhis and the "Nava Nidhi" or the nine types of wealth.

Forms of Siddhis in Bhagavata Purana
In the Bhagavata Purana, Lord Krishna describes that there are Ten Secondary Siddhis. These are;

1. Anurmi-Mattvam means being undisturbed by hunger, thirst, and other bodily disturbances.
2. Dura-Sravana helps in hearing things far away.
3. Dura-Darsanam leads to seeing things far away.
4. Manah-Javah is one of the secondary Siddhi which helps in moving the body wherever thought goes.
5. Kama-Rupam assists in assuming any form desired.
6. Para-Kaya Pravesanam is the Siddhi or the power of entering the bodies of others.
7. Sva-Chanda Mrityuh means the control to die when one desires.
8. Devanam Saha Krida Anudarsanam means witnessing and participating in the amusement of the apsaras.
9. Yatha Sankalpa Samsiddhih is the form of secondary Siddhi which means perfect accomplishment of one`s determination.
10. Ajna Apratihata Gatih which signifies orders or commands being unhindered.

Siddhis of Yoga and Meditation
There are Five Siddhis of Yoga and Meditation mentioned in the Bhagavata Purana. They are "Tri-Kala-Jnatvam" which means the knowledge of the past, present and future; "Advandvam" or the tolerance of heat, cold and other dualities; "Para-Citta Adi Abhijnata" signifies knowing the minds of others; "Agni-Arka-Ambu-Visa Adinam Pratistambhah" means checking the influence of fire, sun, water and poison; and "Aparajayah" means remaining unconquered by others.

In the Vibhuti Pada of the Yoga Sutras of Patanjali a comprehensive instruction is given for obtaining various forms of Siddhis. The specific form of Siddhis is attained through properly aimed "Samyama". Samyama is the united practice of Dharana, Dhyana and Samadhi.

Thus, Siddhis can be obtained by meditation, control of the senses, devotion, herbs, mantras, practice of Pranayama or good birth.