In a 1927 letter to Sigmund Freud, Romain Rolland coined the phrase "oceanic feeling" to refer to "a sensation of 'eternity'", a feeling of "being one with the external world as a whole", inspired by the example of Ramakrishna, among other mystics.

According to Rolland, this feeling is the source of all the religious energy that permeates in various religious systems, and one may justifiably call oneself religious on the basis of this oceanic feeling alone, even if one renounces every belief and every illusion. Freud discusses the feeling in his Civilization and Its Discontents (1929). There he deems it a fragmentary vestige of a kind of consciousness possessed by an infant who has not yet differentiated itself from other people and things.

In November 1927, Freud's new book The Future of an Illusion was printed, and one of the copies was sent by him to Rolland. Rolland responded with a letter to Freud, writing that he should also consider spiritual experiences, or "the oceanic feeling", in his future psychological works:

Mais j'aurais aimé à vous voir faire l'analyse du sentiment religieux spontané ou, plus exactement, de la sensation religieuse qui est [...] le fait simple et direct de la sensation de l'éternel (qui peut très bien n'être pas éternel, mais simplement sans bornes perceptibles, et comme océanique).

But I would have liked to see you doing an analysis of spontaneous religious sentiment or, more exactly, of religious feeling which is [...] the simple and direct fact of the feeling of the eternal (which can very well not be eternal, but simply without perceptible limits, and like oceanic).

— Letter from Rolland to Freud, December 5, 1927

Rolland based his description on the example of Ramakrishna who had his first spiritual ecstasy at the age of six. From his 10th or 11th year of school on, the trances became common, and by the final years of his life, Ramakrishna's samādhi periods occurred almost daily.

Rolland described the trances and mystical states experienced by Ramakrishna and other mystics as an "'oceanic' sentiment", one which Rolland had also experienced. As described by Rolland, it is "a sensation of 'eternity', a feeling as of something limitless, unbounded", a "feeling of an indissoluble bond, of being one with the external world as a whole". Rolland believed that the universal human religious emotion resembled this "oceanic sense". In his 1929 book The Life of Ramakrishna, Rolland distinguished between the feelings of unity and eternity which Ramakrishna experienced in his mystical states, and Ramakrishna's interpretation of those feelings as visions of the goddess Kali figure.