The document defines key terms related to open sentences including:
- Open sentence: A mathematical statement with one or more variables
- Solution: The replacement value for a variable that makes an open sentence true
- Replacement set: The set of numbers that can replace a variable
- Solution set: The set of elements from the replacement set that make an open sentence true
Examples are provided to demonstrate finding the solution set of equations and inequalities given a replacement set. The document notes that solution sets do not have to be numbers and can include other types of values.