The actual definition of a crucible is a  "vessel made of a refractory substance such as graphite or porcelain, used for melting and calcining materials at high temperatures." In other words, it is something that can withstand very high temperatures used to melt things inside of it.

The title "The Crucible" can be seen as a metaphor for what happens in Salem in many ways. "A place that influences you" is a definition of The Crucible because, like metals can be melted, the people who were accusing others of witchcraft were trying to "melt" the resolve of those who were accused. Salem itself can be seen as the vessel that could withstand high temperatures, and the people who were accused were those who were influenced.

One could also say that those who were caught up in the hysteria of the accusations were influenced. Reverend Parris initially rejects the idea of witchcraft because he does not want people to associate his family with such devilish acts. However, the Putnams soon convince him that witchcraft is involved, and that the best thing he can do for his reputation is to address it head-on and get prosecute those who are involved. Thus, he is "melted," or influenced, and feeds the hysteria and accusations.

crucible: A place, time, or situation characterized by the confluence of powerful intellectual, social, economic, or political forces (American Heritage Dictionary)