There is no specific shared definition of nexus across the US, but most of the time states consider that a “physical presence” or “economic connection” creates nexus.
A phisical presence in a state may occur when a merchant:
leaves or works in this state (has a store, office, or distribution center)
has an employee, who leaves or works in this state
has an inventory warehouse in this state
has an affiliate in this state
has temporary sales in this state on trade shows, craft fairs, etc.
An economical connection is an economic activity in a state, regardless of whether a merchant has a physical presence in that state, and it may occur when a merchant:
exceeds a certain threshold on the volume of sales in this state
exceeds a certain threshold of the number of transactions in this state
To determine whether you have nexus in a state please consult your accountant or local tax authority.
If a merchant has sales tax nexus in a state, then this merchant must collect sales tax from buyers in that state. This means a merchant must determine the sales tax rate in that state, plus any local sales tax (a county, city, and “special taxing district” taxes) that might apply.
In order to collect sales tax legitimately a merchant must first of all register for a sales tax permit in that state and then file and report sales tax returns in that state.