An expert in a particular field is generally defined by "the person has a high degree of knowledge about a topic." This expertise typically comes from extensive study, research, and practical experience in the field, which often also leads to publishing articles and books, working at the same job or within the same field for a long time, and obtaining advanced degrees. However, the fundamental characteristic that defines an expert is their deep, comprehensive understanding and ability to apply their knowledge in practice, not just formal education or years of employment.
