Essentially, it means a thick blade with lots of niku. At least that's what it means these days.
It is said that what is called the Spirit of an Age is something to which one cannot return. That this spirit gradually dissipates is due to the world's coming to an end. In the same way, a single year does not have just spring or summer. A single day, too, is the same. For this reason, although one would like to change today's world back to the spirit of one hundred years or more ago, it cannot be done. Thus it is important to make the best out of every generation.- hagakure