the quality of being difficult to bend, cut, or break

These alloys are characterized by their extreme hardness. 这些合金的特点是具有极高的硬度。

Most international golf players own 40 or 50 different balls of varying hardness, weight, and bounciness. 大多数国际高尔夫球手拥有40或50个不同硬度、重量和弹性的球。

the quality of being difficult to understand, do, experience, or deal with

The hardness of his life stemmed from the chronic poverty into which he was born. 因为他生于长期贫困的家庭，所以生活艰难。

There was warmth and repose enough there to set against the hardness of the day's demands. 那里有足够的温暖和宁静，足以应付艰辛的生活。

the fact of being unpleasant or not at all gentle

For a moment she thought she saw a hint of humour softening the hardness of his eyes. 有那么一瞬间，她以为她看到一丝幽默软化了他冰冷的眼睛。

He was angered at their hardness of heart. 他对他们的冷酷无情感到愤怒。

(of water) the fact of containing a high level of minerals that prevent soap from making bubbles

This plant is not fussy about water hardness, but does best in soft water. 这种植物对水的硬度不太在意，但是在较软的水里长得最好。

Calcium and magnesium are the two most common minerals responsible for the hardness of water. 钙和镁是造成水质硬的两种最常见的矿物质。