horizontal

Compared meaning: level and without a slope (adjective).

比较义项：平坦、水平的（形容词）。

• flat

  Flat focuses on smoothness: no bumps, curves, or wrinkles on a surface, regardless of which way it faces — a flat surface can even be vertical.

  flat强调平滑：表面没有凹凸、弯曲或褶皱，跟朝向无关——一个平坦的表面甚至可以是竖直的。

  The tabletop was perfectly flat, with no bumps or dips.

  桌面非常平坦，没有一点凹凸不平。

• horizontal 本词

  Horizontal focuses on direction: parallel to the ground and level from side to side, as opposed to vertical or upright — orientation matters more than smoothness.

  horizontal强调方向：与地面平行、左右水平，跟竖直或直立相对——方向比是否光滑更重要。

  Draw a horizontal line across the page before you start writing.

  在开始写字之前，先在纸上画一条水平线。

Flat and horizontal both describe something without a slope. Flat means smooth, with no bumps or curves, no matter which way a surface faces. Horizontal means parallel to the ground, not vertical, so direction matters more than smoothness. Flat's 'apartment' meaning is not compared here.

这两个词都可以形容表面或物体“没有起伏、不倾斜”，但侧重点不同：flat强调表面光滑、没有凹凸或褶皱，跟朝向无关；horizontal强调与地面平行、跟“竖直”相对，侧重方向。这里不比较flat表示“公寓”（英式英语用法）等其他意思。