What the word means, no doubt, know everything. Every day at school you hear: "the Topic of today's lesson...", "next time we repeat topic...", "thesis, which you will write..."
And in music, the term "subject" takes on a different meaning. There you can hear: "the Subject of hostility...", "the Main theme of the first part...", "the Main theme of the finale...", "Subject Maiden...".
What is the meaning of this musical term?
The theme music is like a hero of music, his character. From the point of view of musical technology is a building that can serve as a basis for development. Therefore, composers tend to write themes, vivid characters, expressive and memorable.
There are such forms of musical works that use only one theme. As happens, for example, in the Fugue (if it is normal, and not double or triple Fugue - in those, respectively, two and three subjects), in variations. Often several of them. For example, in the first movement of a Sonata or Symphony is the main and side of the party, each of which has a separate theme, and sometimes no one (you can read about this in the story of the Sonata). In the following parts of the Symphony too several those two or three, and in the final could be much more. Topics may be extended, expanded, can be short and concise. But anyway, the topic should be relatively complete build. Sometimes the concept of the theme coincides with the concept of leitmotif. It is in this sense usually says: "the Theme of love...", "theme of the struggle for happiness", "the Theme of der ring des Nibelungen (the leitmotifs of you probably already read in the previous pages).