水平線的意思

straight lines whose slope (or gradient, 斜率) are 0 are so called horizontal lines
(水平線)

in other words,
for straight lines that can be expressed in the form y=c (c is a real constant) on a Cartesian plane, they are horizontal lines

水平線指地面與天空的分隔線，如果身處海中的船上，則可以輕易看到水平線

水平線或地平線或海平線(Horizon)==是指當地球或海洋和天空似乎相遇著時成為一條線的景象,那條線就被稱為地平線。