What part of speech is decline?

"Decline" can be several parts of speech, depending on how it's used in a sentence:

* Verb: "The economy is declining." Here, decline is the action being performed.

* Noun: "There was a decline in sales last month." Here, decline is the thing that is happening.

* Adjective: "The declining population of the city is a concern." Here, declining describes the population.

To determine the part of speech, consider the context of the sentence and how the word functions within it.