Key phrases, key terms, key segments or just keywords are the terminology which is used for defining the terms that represent the most relevant information contained in the document. Although the terminology is different, function is the same: characterization of the topic discussed in a document. The task of keyword extraction is an important problem in text mining, information extraction, information retrieval and natural language processing (NLP).
Keyword assignment vs. extraction
Keyword assignment methods can be roughly divided into:
- keyword assignment (keywords are chosen from controlled vocabulary or taxonomy) and
- keyword extraction (keywords are chosen from words that are explicitly mentioned in original text).
Methods for automatic keyword extraction can be supervised, semi-supervised, or unsupervised. Unsupervised methods can be further divided into simple statistics, linguistics or graph-based, or ensemble methods that combine some or most of these methods. 
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- Beliga, Slobodan; Meštrović, Ana; Martinčić- Ipšić, Sanda. (2014). Toward Selectivity-Based Keyword Extraction for Croatian News (PDF). Surfacing the Deep and the Social Web (SDSW 2014). Vol. 1310. Italy: CEUR Proc. pp. 1–14.
- Alrehamy, H.; Walker, C. (2017). SemCluster: Unsupervised Automatic Keyphrase Extraction Using Affinity Propagation. 17th UK Workshop on Computational Intelligence.
- "Keyword Extraction: from TF-IDF to BERT".
- Tayfun Pay; Stephen Lucci (2017). Automatic Keyword Extraction: An Ensemble Method. 2017 IEEE International Conference on Big Data (Big Data). doi:10.1109/BigData.2017.8258552.