In most jurisdictions, nolle prosequi is a dismissal of the case at the request of the prosecutor, and therefore is not a conviction and the true answer would generally be no, you do not have a conviction. However, state law and practice does vary, and my answer could be wrong under the law of your state. To be certain you should ask the advice of a Maryland attorney.
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Legal disclaimer: Mr. Tabaku may be reached at (240) 750-4663 or firstname.lastname@example.org. Mr. Tabaku is an attorney licensed in the state of Maryland. This answer is for general information only based upon the facts stated in the question and does not create an attorney client relationship between Mr. Tabaku or Tabaku Law Firm, LLC and any person. You should schedule a consultation with an attorney to discuss the specifics of your legal issues.