* Adjective: This is the most common use, describing something that exists in thought or as an idea but not having a physical or concrete existence.
* Noun: This is used to refer to the concept of an abstract idea itself. For example, "The abstract of the paper was very clear."
* Verb: This is less common, meaning to summarize or extract the main points of something. For example, "She abstracted the key points from the long report."
So, to determine the part of speech, you need to consider how it's being used in the sentence.