Chapter 7 - Algebra: Graphs, Functions, and Linear Systems - 7.2 Linear Functions and Their Graphs - Concept and Vocabulary Check - Page 460: 7

horizontal

RECALL: The graph of a line whose equation is $y=k$ is a horizontal line whose y-intercept is $k$. The given line $y=3$ has $k=3$. Thus, its graph is a horizontal line whose y-intercept is $3$. Therefore, the missing expression in the given statement is: horizontal