According to the Oxford English Dictionary, a Miasma is a term used to describe “an unpleasant or unhealthy atmosphere.” It can also mean a really foul stench or odour.
However, I love the literary meaning which says that the word is used to describe a sense of foreboding. This basically means an atmosphere that predicts something bad is going to home, stereotypically this is portrayed literally by stormy weather and dark clouds.
The term derives, unchanged, first from the Modern Latin word meaning “noxious gases”, which itself came from the Greek, meaning something that is “polluted” or “tainted by guilt”. The meaning of the word certainly hasn’t changed, at all, throughout history, and the word has been in existence since the mid-17th Century.