A Publican is a Tax Collector. Here is a comparison between the King James Version which uses the word Publican and the equivalent in the New King James Version:
Luke 18:10 KJV Two men went up into the temple to pray; the one a Pharisee, and the other a publican.
Luke 18:10 NKJV Two men went up to the temple to pray, one a Pharisee and the other a tax collector.
I hope you have found this helpful.