The term "spirit" is in many cases used to describe a sense of presence of any kind.

"Spirit" or soul means "essence". The spirit is the collection of one's experiences, personality, attitudes, and knowledge that is not physically in our body. One can say that the spirit is a collective term for everything in our body that cannot be physically grasped and measured.

Some believe that the soul can physically separate from our body and that the spirit or soul can survive the decay of the body. Therefore, the spirit is believed to be something that can survive the body's death.
We have all heard of the concept of good and evil spirits. Actually, it's like good and evil people - with the qualities they might have. For a long time people have been working on registering spirits in many different ways. Eg. using advanced measuring equipment or through photographs. Some people claim to be in direct contact with spirits.

It is said that a spirit can appear in many different ways. It can be like a complete physical person, completely invisible, a fog, or a luminous ball.

Fog depicting alleged spirit activity at Goodleberg Cemetary / Wikipedia / Creative Commons