So any time you have two atoms bonding, the bond axis is just the axis that they're bonding along.

任何时候如果你有两个原子成键，键轴就是它们成键的方向。

麻省理工公开课 - 化学原理课程节选

And a sigma bond forms any time you have two orbitals coming together and interacting on that internuclear axis.

当你把两个轨道合在一起,并在核间轴上有相互作用时,就形成了sigma键。

麻省理工公开课 - 化学原理课程节选

So in terms of the carbon hydrogen bond, it's a sigma bond, - because we define it -- any time we are bonding to an atom, we have to keep redefining our bond axis to whatever two atoms we're talking about.

对于碳氢键，它是sigma键，因为我们定义它-,任何时候我们有原子成键，我们要重新定义键轴方向,为我们所讨论的两个原子方向。

麻省理工公开课 - 化学原理课程节选