If a dictionary (about the use of scientific terminology) includes snippets from several sources, would an authorization be needed from each copyright holder?
Each entry in the dictionary would include several quotes. And multiple quotes would come from concrete copyright holders. However, no entry would include multiple quotes from the same source. And no quote would be longer than 2-3 sentences. More than 80-90% of the dictionary would be quoted material.
There's a dictionary that does this already - it's the Collins Cobuild dictionary, which uses corpora to provide examples for its definitions.
Generally the text uses no more than is necessary to convey the meaning.
Whether it falls under fair use will depend on the amount of a text used and other factors, such as the effect on the original text's value. The purpose is also considered, and the advancement of knowledge through education is usually accepted.
Whether it falls under fair dealing (relevant to the United Kingdom and other Commonwealth countries) will generally be determined by the amount that you use, and the purpose of the use - illustrative teaching uses (such as examples in dictionaries) usually fall into this category.
