Primus Pilus was a title given to the highest-ranking centurion in the Roman army. Although it has no exact synonym, there are some words and phrases that can be used to describe the importance and authority associated with this position. The closest synonym for Primus Pilus could be 'chief centurion' or 'first centurion'. Other options include 'senior centurion', 'commanding centurion', 'lead centurion', or 'top-ranking centurion'. The Primus Pilus was known for his skill and experience in training and leading soldiers, hence some words that could possibly describe him could be 'skilled trainer', 'seasoned commander', 'battalion leader', or 'tactical expert'.