If the line is horizontal, it is the greek letter theta, often used to label an angle.
If the line is diagonal, it is used to denote the diagonal of a circle. As the greek letter phi it is used to denote an empty set and also used to label an angle.
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Izzy and Amie made the zero with a line through it!
How about "is always zero"?
A straight line passes through the x-axis one time, if its slope is non-zero, or not at all, if its slope is zero.
The slope is zero.
A line with slope of zero is horizontal. A line with no slope is vertical because slope is undefined on a vertical line.