Expertise has two different, but related meanings. It either refers to expert knowledge or the know-how, skills, or expertise in something. I.E. it either refers to knowing-that or knowing-how.
With knowing-that, think about an expert opinion witness. They provide their professional testimony because they know particular facts.
When thinking about knowing-how, think about professions like a mechanic. They have the skills informed by knowledge to do a task that someone without that skill and knowledge would not be able to do.
The type of expertise you are talking about will change who will be considered an expert.
There is a lot of debate over what makes an expert. This partly stems from the two-part definition mentioned above, but largely because expertise looks differently in different contexts. Various disciplines may value different characteristics of expertise.
There are also various ways for a person to gain expertise and/or prove their expertise.
Study a subject for a long time (How long is long enough? It depends).
Is credentialed (Does simply having a Ph.D. in a subject make you an expert? Maybe)
Experience an event firsthand
Can communicate their knowledge effectively (Have they published? Can people actually understand what they are trying to say? These are important in proving someone's expertise, though it doesn't necessarily make someone an expert).
Information is created through a process. The type of information that you encounter within days of an event will be different than the type of information included in sources written months or years after the event has occurred.
Video brought to you by Rebecca Crown Library at Dominican University.