Final answer:
A boolean expression is considered 'linear' if each variable in the expression only occurs once, simplifying logical analysis and design of digital circuits.
Step-by-step explanation:
A boolean expression is considered linear if each variable in the expression occurs only once. In Boolean algebra, expressions are used to create logical statements that reflect various conditions that can be either true or false. A linear Boolean expression simplifies the process of logical analysis and design of digital circuits because there's no need to consider the cases where a variable might interact with itself.
For example, the expression A AND B OR C is linear because A, B, and C each occur once. However, the expression A AND B OR A is not linear because the variable A occurs twice.