definition expertise
The definition of expertise is advanced knowledge or skill. An example of expertise is a master gardener's knowledge of gardening. the skill, knowledge, judgment, etc. of an expert.
An expert, more generally, is a person with extensive knowledge or ability based on research, experience, or occupation and in a particular area of study. Experts are called in for advice on their respective subject, but they do not always agree on the particulars of a field of study.