The thematic word comes from another word: theme. Thus, when we talk about thematic, we are referring to the theme that appears in an exhibition, in a presentation, in a talk or in a myriad of different situations. The term as such can be used both as a noun (for example, when it is said that the theme of a party is to wear pink) and also as a qualifying adjective (for example, when it is said that it is a thematic party, which means that there is a specific topic).
The word subject is synonymous with other words such as question, subject, matter. The topic is what defines, for example, a conversation (if you are talking about a topic of politics or entertainment), a class, an exhibition, a discussion, etc. The theme is then related to the theme since it is in some way or another also a specific theme on which something or someone is exclusively based, not giving rise to other possible themes or themes.
Usually, the concept of thematic or thematic is used to refer to situations, circumstances or events that are characterized by specializing in something or by having characteristic and well-defined elements that are, at the same time, differentiated from others. Thus, it is common to speak, for example, of "theme parks" when talking about entertainment parks that are dedicated to a particular theme (for example, a park dedicated exclusively to history or natural sciences). The parties can also be themed, which means that the decoration, the setting and the participation of the guests will all be related to the chosen theme (for example, a party whose theme is nature). The same happens, for example, with the decoration that a room can have and that can also be themed. Finally, another common example is that of the theme that organizes events, debates, talks or exhibitions and that means that in those situations a specific topic previously defined will be discussed.