Though compound words are multi-syllable in nature, not all words with multiple syllables are compound words. A multi-syllable word is a compound word if it can be broken down into its component words. Consider the word “beautiful.” This word has three syllables but it cannot be divided into distinct meaningful words. So, “beautiful” is not a compound word.
But consider the word “sunshine.” Here, this word can be divided into “sunshine = sun + shine.” The individual words “sun” and “shine” have meanings.
This makes “sunshine” a compound word. So, the best strategy to determine if a given word is compound is to divide it.