Religion: What is a cult?

There are a number of definitions offered at Merriam-Webster Dictionary online.

In recent news, a pastor was criticized for calling Mormonism a cult.

Cult in typical American English usage is definition number 5 in MW:
great devotion to a person, idea, object, movement or work (as a film or book)

For example, the film Blade Runner is considered a cult classic.  In this case, there are fans of the film who really like the film a lot and will buy the "director's cut" of the film and have collector's items associated with the movie and know many details about the film and the making of the film.

Certain musical bands garner a cult following.

Some libertarians would be described as very devoted to the writings of Ayn Rand and that devotion seems cultic to non-devotees.

In these contexts, the term cult is somewhat descriptive and depending on tone of voice may or may not be a disparaging.

Cult in a religious context tends to be disparaging as it usually describes a religious group where usually one leader (or small number) has a charismatic hold over his/her followers and the peer pressure of the group makes it hard for anyone to leave the group.

However, there is a descriptive definition of cult as described by definition number 3 in MW:
a religion regarded as unorthodox or spurious 

There are groups within the major world religions.

For example, in Judaism, there are orthodox, conservative and reform congregations.  The three groups are not identical but they share much in common.

In Christianity, there are Catholic, Orthodox and various Protestant denominations.  Thus, on many (most) doctrinal matters there would be agreement.  Yet, there would be some areas of distinctive.

And so within a given religion, there can be and are groups that hold beliefs that would be considered unorthodox or spurious to the vast majority of the adherents of the orthodox/historical form of that given religion.

This would be the case with Mormonism which is also known as the Church of Jesus Christ of the Latter-Day Saints (LDS).  Mormons use much the same language of traditional Christianity.  However, they call themselves the Latter-Day Saints because they believe that the historic forms of Christianity lost the true message which the LDS believes they have recovered.

Objectively speaking there are three possibilities:  the Mormon claims are correct, the Mormon claims are incorrect or both the Mormon and traditional Christian claims are incorrect.

However, as a matter of intellectually honesty, regardless of the correctness of the claims of either group, is there significant divergence in beliefs between the LDS church and the historic Christian church?

A Catholic priest, Protestant minister and Mormon elder go to have breakfast at a local diner and write down the beliefs of their churches on paper.  What would happen?

The lists would not be identical. 

The Catholic list would have more similarities to the Protestant list than to the Mormon list.

Thus, under MW definition #3, Mormonism is a cult.

However, given that much of the American public has in their minds that cult is definition #5, it is not useful to call Mormonism a cult.

A more precise way to describe the LDS church would be, a religious group that holds some aspects of Christian doctrine yet differs with historic Christan beliefs in a number of key areas.

I heard one Protestant pastor describe his relationship with Mormon friends this way, we have significant theological differences but we work together on issues that matter to our communities and our country.