A person who engages deeply with a small number of powerful individuals in leadership roles and privately challenges/enables them to make better decisions.
Related to some forms of consulting.
As distinct from pundit.
Guru has become a term used derisively in the Internet age because of mis-uses, primarily, and overuse, secondarily. Internet-age objects of derision who socially label as guru typically speak to and purport to help (a) lots of people and (b) people not in leadership roles. I expect the historical definition dating back to ancient India to survive this recent slang meaning.
Per Ventatesh Rao's writings on Guru vs Pundit, Guru is comprable yet symetrically opposed to pundit (Sanskrit "pandat")