Final answer:
Morphemes are the smallest units of meaning in words, which can be standalone words or parts of compound words.
Step-by-step explanation:
The smallest units of meaning in words are known as morphemes. These include both individual words that can stand alone, like "book", and affixes like prefixes or suffixes that modify the meaning of words, such as "un-" in "unhappy". In compound words, like "blackbird" or "dogcatcher", each part of the compound also has meaning; "black" and "bird" each have their own semantic value, and when combined, they create a new word with a related, but distinct, meaning.