Geometry

Question Analysis:
Asks about the definition of y-axis symmetry.

Key Concept Explanation:
Y-axis symmetry means the figure matches itself after reflecting across the y-axis.

Step-by-Step Solution:
Flipping across the y-axis maps the figure onto itself.

Option Analysis:
A. Refers to origin symmetry
B. Correct definition of y-axis symmetry
C. Incorrect — symmetry involves both sides of y-axis
D. Too restrictive ...