It is tempting to start by defining Psychology and Music. The first chapter of Keith E. Stanovich How to Think Straight About Psychology provides a good context about the former and why a simple definition isn't enough (pages 6-13 are particularly useful for our purposes). As for music, here is one definition:
"Music is temporally organized sound and silence, a-referentially communicative within a context. Emphasis is placed on the temporal and communicative aspects of music..."
But does it allow us to understand this, AND, this, AND, this, AND, this, AND ...
Or, as Honing, Cate, Peretz, and Trehub observed:
"Definitional issues are especially problematic because there are no conventional defining criteria of music. Within a culture, people agree, more or less, on what constitutes music, but there is considerably less agreement across cultures."
They suggest we turn our attention to the question of musicality:
"Musicality in all its complexity can be defined as a natural, spontaneously developing set of traits based on and constrained by our cognitive and biological system. Music in all its variety can be defined as a social and cultural construct based on that very musicality. This distinction demarcates two divergent approaches to the cognition and biology of music...
...One approach is to study the structure of music, seeking key similarities and differences in musical form and activity across a variety of human cultures...
...An alternative approach is to study the structure of musicality by attempting to identify the basic underlying mechanisms, cognitive and biological, their function and developmental course, and effective ways to study those mechanisms in human and non-human animals."
Note that musicality as outlined above is not the same as the colloquial use of the word. When we describe someone as very musical we mean (more or less) especially talented in music.
Here is David Huron's critique of the possible answers.
Perhaps we can take an evolutionary perspective:
For next week read Chapter 1 (Introduction) and chapter 2 (Origins of Music) of the textbook (Thompson Music, Thought, Feeling) which further explores this territory. We will discuss the reading next week so come prepared with observations, questions, and ideas. And consult this page about reading and engaging with academic texts (note - American college instructor == British university lecturer)