Samudra (समुद्र) is a Sanskrit term for "ocean", literally the "gathering together of waters" (saṃ- meaning "together" and -udra meaning "water". Dictionary meaning of samudra is ‘confluence’ and ‘ocean/sea’. The word has been borrowed to various languages influenced by Sanskrit, including Modern Indo-Aryan languages such as Hindi समुद्र samudra, Bengali সমুদ্র shômudrô, Gujarati સમંદર samandar, Marathi and Nepali समुद्र samudra, Punjabi ਸਮੁੰਦਰ samuṃdar, and others, like Kannada ಸಮುದ್ರ samudra, Tamil சமுத்திரம் samudraṁ Malayalam സമുദ്രം samudraṁ, Telugu సముద్రం samudram, Burmese သမုဒ္ဒရာ samuddara, Thai สมุทร sàmùt, Khmer សមុទ្រ samout, Lao ມະຫາສະຫມຸດ mahasamud, and Malay samudra.
The precise semantic field of the Vedic word is difficult to establish, and has been much debated, in particular in relation to the question whether the bearers of the Rigvedic culture had direct knowledge of the ocean. Apart from the question of direct acquaintance of the bearers of Vedic culture with the ocean in the modern sense of the word, it is generally accepted that their worldview had the world encircled by oceans, a feature likely inherited from Proto-Indo-European mythology, with a "heavenly ocean" above the world, and a subterranean ocean of the underworld. Varuna was the deity presiding over both these oceans, and over water in general.
From the literal meaning of the term, "Any mass of water more than one drop could be sam-udra: water in a jar, a small pool, a large lake, or the sea". And indeed there are symbolic identifications of small quantities of water with mythical oceans, for example in the famous hymn to Varuna, emphasizing Varuna's omnipresence in every drop of water (AVS 4.16.3 cd)
The oldest vedic commentators like the Brihaddevata of Shaunaka, Nighantu and the Nirukta of Yaska interpret the term Samudra as "ocean". The scholar G.V. Davane studied the occurrences of the term samudra in the Rigveda and concluded that the term means "terrestrial ocean".
The Rigveda also speaks of a western and eastern Samudra (10.136.5-6). And in RV 7.6.7 there is an upper and a lower Samudra, where the upper Samudra seems to be a heavenly ocean.
The Marutas "uplift from the ocean the rain, and fraught with vaporous moisture pour the torrents down" in RV 5.55.5. In RV 9.84.4 the moon (Soma) and the winds stir the Samudra. Additionally, RV 1.48.3 may indicate knowledge of the high tide.
In RV 1.116.4 the Asvins rescued Bhujyu by carrying him for three days and three nights to the sea's farther shore. Thus Samudra seems to refer to the ocean in this verse. There are many other verses in the Rig Veda which refer to this tale (e.g. RV 1.118.6; VI 62, 6; VII 69, 7; VIII 5, 22), and where consequently Samudra could be identified with the ocean as well.
Samudra is usually translated as "ocean, sea"  and the word itself means "gathering of waters". A minority of scholars (e.g. Madhav Deshpande) translate the term as "river". However, the Samudra is never said to flow in the Rigveda, but to receive all rivers (e.g. RV 6.36.3; 8.44.25). The Rigveda also describes the Vedic Sarasvati River as a river that flows to the ocean (e.g. RV 2.41.16-18) and "is pure in her course from the mountains to the sea" (RV 7.95.2). Rigveda 1.71.7 describes the seven great rivers seeking the Samudra and in RV 7.33.8 it is written that all the rivers flow to the Samudra but are unable to fill it.
RV 7.49 says that the Samudra is the eldest of the waters (samudra jyestha), and that the goal of the rivers is the Samudra.
According to M.L. Bhargava  "samudra" stands for a huge inland lake of which there were four or seven in Rgvedic sources. He translates "sagara" with ocean. In this view the lowlands of e.g. Kashmir and Kuruksetra were "samudra", but the sea in which the Ganga fell is a "sagara".
The Vedic deity Varuna is the deity of the ocean (Samudra). The Vedic deity Indra also occurs frequently in connection with the Samudra. The Rigveda narrates that Indra slew the dragon which released the seven rivers and caused them to enter the ocean. Other gods that often occur together with the Samudra are Agni and Soma.
Some scholars like B.R. Sharma hold that the Rigvedic people may have been shipbuilders engaging in maritime trade. In Rigveda 1.25.7; 7.88.3 and other instances, Samudra is mentioned together with ships. In RV 7.89.4 the rishi Vasishta is thirsting in the midst of water. Other verses mention oceanic waves (RV 4.58.1,11; 7.88.3). Some words that are used for ships are Nau, Peru, Dhi and Druma. A ship with a hundred oars is mentioned in RV 1.116. There were also ships with three masts or with ten oars. RV 9.33.6 says: 'From every side, O Soma, for our profit, pour thou forth four seas filled with a thousand-fold riches."
There are other Sanskrit terms in the Rigveda that appear to mean "ocean" or have similar meanings. Among them are the terms salila (ocean depths, unbounded sea), Arnas, Apas (water, sometimes celestial ocean), Purisha (heavenly ocean, clouds, origin of rain - the opposite of Samudra or terrestrial ocean). The waves are called Urmi in the Rigveda, and the lakes are called saras, kula, hrada or hlada. Another term, as mentioned above, is "sagara" (सअगर), which likewise is also found in modern Indo-Aryan languages and languages influenced by Sanskrit as an alternative for "samudra", some even having it more common to use than the latter term, including Balinese, Sundanese, and Javanese (in the form of "segara").
In SB 18.104.22.168 there is (as also in the Rigveda) a reference to an eastern and western Samudra.