In plain English
Physical presence, a location, inventory, employees, sometimes delivery, is the traditional trigger. Many states also apply economic thresholds based on sales into the state.
Thresholds and definitions vary by state and change. Treat nexus as a question to confirm with each state's revenue agency or your accountant, not a rule of thumb.
Example
You open a second warehouse across a state line and now have a registration question in that state.
Why it matters
Uncollected sales tax does not disappear. It becomes a liability the business owes later, with interest.
COMMON MISTAKEAssuming online sales are automatically tax-free.