An employment relationship that either party can end at any time, for any legal reason, without advance notice.
At-will employment is the default in the U.S. (Montana is the only exception). It means the employer can terminate an employee at any time, for any reason that isn't illegal (discrimination, retaliation for protected activity, etc.), without prior notice. The employee can also quit at any time.
Most offer letters explicitly state that the employment is at-will. This is important for employers because it preserves flexibility to respond to business changes, performance issues, or economic conditions. It also means the offer letter and employment relationship don't create a specific term of employment — a letter saying "we're excited to have you start on June 1" doesn't guarantee employment for any particular period.
At-will status can be eroded by actions the employer takes: making promises of continued employment, using progressive discipline that implies process must be followed before termination, or establishing policies that imply a term. Consistency in applying at-will policies matters. Employment law in this area is nuanced, and HR practices that inadvertently limit at-will status are a common source of wrongful termination claims.