What is a "thought?" (Particularly in reference to thoughts that arise during meditation)
According to Buddhism thought to arise from the consciousness. Which are called Vitakka and Vicara. There are many types of wholesome and unwholesome thoughts. A person should have a good grasp of Abhidhamma to understand the differences completely.

Abhidhamma in practice:
http://103.242.110.22/theravadins/English-articles/abhidhamma-in-practice.pdf