There is a very important concept called as Inverted Index in Elasticsearch. The Lucene text processing library is used in Elasticsearch for breaking down raw text into a series of terms. Once terms are broken these are stored in what is known as Inverted Index. The figure (Figure 07) shows an Inverted Index, which is in turn is a map data structure with key as term and value as list of document ID’s where this term exists in the document. Now you know why it is called Inverted Index, because in this data structure we have term as key and value as ID:

Figure 07: Document and Index (Inverted Index)