Although the idea of an intelligent model encoding real-world “things, not strings” and their (inter-)relationships was 
already  present  in  the  literature  since  the  ‘80s [9],  the  term  knowledge  graph  has  regained  popularity  since  Google  an-
nounced the their Knowledge Graph in 2012.3 Yet, there does not seem to be a precise deﬁnition for the term knowledge 
graph even to date [24]. The general agreement is to consider a knowledge graph as a data structure describing entities and 
their relationships by means of a directed, edge-labelled graph, often organising them in an ontological schema, and also 
covering several topics. One refers to the set of concepts and properties between them as Terminology Box (TBox) and to 
the set of statements about individuals belonging to those concepts as Assertion Box (ABox). With the increasing amount of 
techniques to scale up the Web, knowledge graphs are now result of the integration, extraction and manipulation of data
Knowledge graphs as tools for explainable machine learning: A survey