You can learn terminology related to specific areas of knowledge ('AOK's') by reading wikis and online encyclopedias. This is because exploring any AOK always involves learning terminology.
There are topical dictionaries which focus on a particular AOK or small group of interrelated of AOKs. (e.g. Dictionary of Gems and Gemology)
Are there any online dictionaries that allow users to select a topic and see a comprehensive list of its related terminology?