There is no single definition that describes all the parts of music theory. As a practical discipline, music theory is concerned with the fundamentals of how music is made, and also with analysis of sonic phenomena, including silence, and how they relate to music.
For the beginner, music theory is the study and descriptions of music rudiments required to notate and read music, such as key signatures, time signatures, rhythmic notation, pitches (notes), scales, modes, chords, tensions, and elements of composition such as rhythm, harmony, and counterpoint.
Modern music theory textbooks often include descriptions and analysis of practical applications of the harmonic series, musical acoustics, orchestration, improvisation, scales, tonal and rhythmic relationships, performance, and electronic production.
While this might seem like a lot of information, beginners should focus on just the fundamentals, which limits the scope and make it easier to forge ahead.
Think about music theory as analysis that occurs after the music happens.
Analyzing a piece of music doesn’t change the actual music at all. Beginning theory includes learning the terms we use to describe what happened in a piece of music.
Whatever we decide to call something, the music is the music and it doesn’t change because of how we analyze it. This fact is highlighted by the varying ways different cultures and languages choose to describe music theory.
For example, I have many students from other countries who are non-native English speakers. Their music theory knowledge might be strong in their own language, but when they come to study in the US they are challenged at first by learning our terminology.
This is normal, and it’s important to note that music theory is always rooted in a time and place. A Musicologist might choose to study music theory based on origins instead of focusing on individual works.