According to Wikipedia

Open data are data that are openly accessible, exploitable, editable and shareable by anyone for any purpose. Open data are generally licensed under an open license.[1][2][3]

What is its purpsose?

The goals of the open data movement are similar to those of other “open(-source)” movements such as open-source software, open-source hardware, open content, open specifications, open education, open educational resources, open government, open knowledge, open access, open science, and the open web. The growth of the open data movement is paralleled by a rise in intellectual property rights.^[4] The philosophy behind open data has been long established (for example in the Mertonian tradition of science), but the term “open data” itself is recent, gaining popularity with the rise of the Internet and World Wide Web and, especially, with the launch of open-data government initiatives Data.gov, Data.gov.uk and Data.gov.in.

Open data can be linked data—referred to as linked open data

Example:

One of the most important forms of open data is open government data (OGD), which is a form of open data created by ruling government institutions. The importance of open government data is born from it being a part of citizens’ everyday lives, down to the most routine and mundane tasks that are seemingly far removed from government.^{[citation needed]}

The abbreviation FAIR/O data is sometimes used to indicate that the dataset or database in question complies with the principles of FAIR data and carries an explicit data‑capable open license.

Alternate definitions

One more definition is the Open Definition which can be summarized as “a piece of data is open if anyone is free to use, reuse, and redistribute it—subject only, at most, to the requirement to attribute and/or share-alike.
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 Open Data Institute‘s

open data is data that anyone can access, use or share,