Sānanda samādhi, also known as "supreme bliss", or "with ecstasy", is the third level of the four samādhi described in the Yoga Sutras 1:17 by Patanjali. Sānanda: sa- means “with”, ananda means joy, bliss.
Historical descriptions 
In Sutra 1:17 Patanjali tells us that samprajnata samadhi comprises four stages: "Complete high consciousness (samprajnata samadhi) is that which is accompanied by vitarka (reasoning), vicara (reflection), sananda (ecstasy), and sasmita (a sense of 'I'-ness)." Sānanda is free from vitarka and vicara.
This is joyous samādhi and it gives intense joy. In sānanda the gross and the five elements are given up. There arises in the Yogi a peculiar perception in the form of intense joy. In this state the mind continues to function, but knowledge of any particular object slowly fades away and the yogi becomes aware of the inner consciousness only. He experiences supreme bliss. In sānanda samādhi the yogi experiences a state of rapture or ecstasy, and the only thought in the mind is the wordless awareness of the feeling of "I am in pleasure, I am happy."
Modern descriptions 
According to Dr. Sarasvati Buhrman PhD, "Babaji once explained that when people feel blissful sensations during sādhanā, on a gross level the breath is equal in both nostrils, and on the subtle level pranic flow in ida and pingala nadis is balanced. This is called the sushumna breath because the residual prana of the sushuma, the kundalini, flows in sushumna nadi, causing sattva guna to dominate. "It creates a feeling of peace. That peace is ananda." In sananda samadhi the experience of that ananda, that sattvic flow, is untainted by any other vrittis, or thoughts, save the awareness of the pleasure of receiving that bliss."
According to John C Lilly, sānanda is the highest state of consciousness that can be experienced while staying in the body. In his book Center of the Cyclone he describes sānanda as, "blissful state, making the Christ, the green qutub, realization of baraka, the reception of divine grace, cosmic love, cosmic energy, heightened bodily awareness, highest function of bodily and planetside consciousness, being in love, being in a positive LSD energy state. In the Oth emotional center in the chest."