Guru

n.capable, thoughtfulplayful

What it means

A respected expert who offers knowledgeable guidance.

Reach for it when referring informally to a highly respected expert who teaches or advises

The gardening guru showed beginners how to restore poor soil.

warm
mentor
ongoing guidance develops another person's ability
not when A mentor need not be the field's leading technical authority.
professional
adviser
practical recommendations are the person's main role
not when An adviser may synthesize others' knowledge without being a master of the subject.
professional
expert
verified knowledge or skill is the relevant qualification
not when Expert is evidence-based and neutral but does not imply teaching, influence, or personal following.
professional
specialist
narrow, trained competence should be stated precisely
not when Specialist is more bounded than guru and carries no claim of broad influence.
formal
authority
recognized command of a subject is central
not when Authority can also mean institutional power and may sound unquestionable.
literary
master
exceptional command of a craft deserves elevated praise
not when Master can sound hierarchical, gendered, or inflated when competence is merely strong.

marketing guru
The marketing guru showed the cooperative how to test claims honestly.
tech guru
A local tech guru repaired the network and documented every step.
self-styled guru
The self-styled guru offered confidence but no verifiable credentials.

Guru is a spiritual teacher in Hindu, Buddhist, Sikh, and Jain traditions. Casual expert language can trivialize that religious role.

The definition and the example on this page are written here. The dictionaries above ground which meaning is being described, not the wording.