What is Dam?

Legal Definition
A dam is a barrier that impounds water or underground streams. Reservoirs created by dams not only suppress floods but also provide water for activities such as irrigation, human consumption, industrial use, aquaculture, and navigability. Hydropower is often used in conjunction with dams to generate electricity. A dam can also be used to collect water or for storage of water which can be evenly distributed between locations. Dams generally serve the primary purpose of retaining water, while other structures such as floodgates or levees (also known as dikes) are used to manage or prevent water flow into specific land regions.

The word dam can be traced back to Middle English, and before that, from Middle Dutch, as seen in the names of many old cities. The first known appearance of dam stems from 1165. However, there is one village, Obdam, that is already mentioned in 1120. The word seems to be related to the Greek word taphos, meaning "grave" or "grave hill". So the word should be understood as "dike from dug out earth". The names of more than 40 places (with minor changes) from the Middle Dutch era (1150–1500 CE) such as Amsterdam (founded as 'Amstelredam' in the late 12th century) and Rotterdam, also bear testimony to the use of the word in Middle Dutch at that time.
-- Wikipedia
Legal Definition
A construction of wood, stone, or other materials, made across a stream of water for the purpose of confining it; a mole.

2. The owner of a stream not navigable, may erect a dam across it, and employ the water in any reasonable manner, either for his use or pleasure, so as not to destroy or render useless, materially diminish, or affect the application of the water by the proprietors below on the stream. He must not shut the gates of his dams and detain the water unreasonably, nor let it off in unusual quantities to the annoyance of his neighbors. 4 Dall. 211; 3 Caines, 207; 13 Mass. 420; 3 Pick, 268; 2 N. H. Rep. 532; 17 John. 306; 3 John. Ch. Rep. 282; 3 Rawle, 256; 2 Conn. Rep. 584; 5 Pick. 199; 20 John. 90; 1 Pick. 180; 4 Id. 460; 2 Binn. 475; 14 Srrg. & Rawle, 71; Id. 9; 13 John. 212; 1 McCord, 580; 3 N. H. Rep. 321; 1 Halst. R. 1; 3 Kents Com. 354.

3. When one side of the stream is owned by one person and the other by another, neither, without the eonsent of the other, can build a dam which extends beyond the filum aqua, thread of the river, without committing a trespass. Cro. Eliz. 269; 12 Mass. 211; Ang. on W. C. 14, 104, 141; vide Lois des Bat. P. 1, c. 3, s. 1, a. 3; Poth. Traite du Contrat de Societe, second app. 236; Hill. Ab. Index, h. t.; 7 Cowen, R. 266; 2 Watts, R. 327; 3 Rawle, R. 90; 17 Mass. R. 289; 5 Pick. R. 175; 4 Mass. R. 401. Vide Inundation.
-- Bouviers Law Dictionary
Legal Definition
A construction of wood, stone or other materials, made across a stream for the purpose of penning back the waters. This word'is used in two different senses. It properly means the work or structure, raised to obstruct the flow of the water in a river; but, by a well-settled usage, it is often applied to designate the pond of water created by this obstruction. Burnham v. Kemp-ton, 44 N. H. 89; Colwell v. Water Power Co., 19 N. J. Eq. 248; Mining Co., v. Hancock, 101 Cal. 42, 31 Pal12
-- Black's Law Dictionary