1. A natural language-relatedness (NLR) search tool, comprising: one or more memory storage devices storing respective computer-executable instructions for a semantic analyzer, a concept expander, and a concept space generator; and one or more processors configured to: obtain a candidate term via a user interface of the NLR search tool, the candidate term comprising one or more natural language words; execute the respective computer-executable instructions stored on the one or more memory storage devices for the semantic analyzer, thereby causing the NLR search tool to operate, using at least one of an explicit semantic analysis or an implicit semantic analysis, on the candidate term to discover an initial subset of a set of concepts of a digital corpus, each concept of the initial subset of concepts being semantically associated with the candidate term, the digital corpus comprising a plurality of digital articles that are related in at least one of topic, subject matter, structure, syntax, or format, and each digital article including natural language therein, and the initial subset of concepts that corresponds to the candidate term and to the digital corpus being an output of the semantic analyzer; execute the respective computer-executable instructions stored on the one or more memory storage devices for the concept expander, thereby causing the NLR search tool to: mine a set of concept association rules from the digital corpus, including: for each candidate rule corresponding to the set of concept association rules, (i) determine a respective measure of support based on a number of occurrences, in a set of transactions of the digital corpus, of a set of antecedent concepts of the each candidate rule together with a set of consequence concepts of the each candidate rule; and (ii) determine a respective measure of confidence based on the respective measure of support and a number of occurrences, in the set of corpus transactions, of the set of antecedent concepts of the each candidate rule; and determine a set of candidate rules as the set of concept association rules, a size of the set of concept association rules limited based on a set of rule-limiting parameters, and the set of rule-limiting parameters including at least one of: a number of concepts included in the set of consequence concepts, a minimum strength of the respective measure of support, or a minimum strength of the respective measure of confidence; receive, as an input, the output of the semantic analyzer including the initial subset of concepts corresponding to the candidate term and to the digital corpus; and operate, using the set of concept association rules, on the initial subset of concepts that corresponds to the candidate term and to the digital corpus to discover an expansion subset of concepts of the digital corpus, each concept of the expansion subset of concepts being semantically associated with the candidate term; execute the respective computer-executable instructions stored on the one or more memory storage devices for the concept space generator, thereby causing the NLR search tool to generate a concept space of the candidate term, the concept space including at least a portion of the initial subset of concepts corresponding to the candidate term and the digital corpus and at least a portion of the expansion subset of concepts corresponding to the candidate term and the digital corpus; search, using the generated concept space, the digital corpus for a first set of digital articles corresponding to the at least the portion of the initial subset of concepts of the generated concept space and a second set of digital articles corresponding to the at least the portion of the expansion subset of concepts of the generated concept space; retrieve, from the digital corpus, at least a portion of each digital article included in the second set of digital articles corresponding to the at least the portion of the expansion subset of concepts of the generated concept space; and display, at the user interface of the NLR search tool, the retrieved at least the portion of the each digital article included in the second set of digital articles corresponding to the at least the portion of the expansion subset of concepts of the generated concept space.