The CDC has updated guidance for people who are moderately to severely immunocompromised: 

Moderately to severely immunocompromised people aged ≥18 years who received a 2-dose mRNA primary series and an additional mRNA additional dose (three total mRNA vaccine doses) are eligible for a single COVID-19 booster dose (Pfizer-BioNTech, Moderna, or Janssen)—a fourth dose—at least six months after completing their third mRNA vaccine dose.  

For these people, the three mRNA vaccine doses are considered the “primary series” of the vaccination, and the fourth dose is considered a “booster.”  

More information on terminology and dosage can be found here.