Here's how it works:
* The sentence will often use words like "that is", "or", "in other words", "means", "refers to", "also known as", etc. to introduce the definition.
* The definition of the unknown word will be directly stated within the same sentence.
Example:
"The ornithologist, that is, a person who studies birds, was fascinated by the rare species."
In this example, "that is" introduces the definition of "ornithologist" as a person who studies birds.