What is the definition of a “hostile work environment”?

The definition of "hostile work environment" varies slightly by the type of discrimination claim that a person is intended to pursue.  In many cases, if refers to an offense environment directed towards an employee as part of ongoing sexual discrimination.  For example, when a supervisor requires sexual favors from a female manager in exchange for promotional opportunities.  However, "hostile work environment" has also been extended to situations where an employer allows one employee to treat another employee in a sexually offensive manner.  Some examples from Pennsylvania courts include "grinding another employee from behind", making sexually inappropriate requests, or touching the other employee in private areas without their consent.  Generally one comment will not constitute a hostile work environment.  Instead, the courts will look to the overall pervasiveness of the conduct-- i.e. how often it occurs and the extent of the conduct. 

Before someone can tell you whether you are a victim of a "hostile work environment" for a discrimination complaint, they would need more information on two other factors.  The first is the nature of the conduct directed towards you.  You mention that the notes are "creepy", however, creepy doesn't necessarily qualify for sexual harassment.  The creepy acts must encompass something more sexual or discriminatory.  The second factor is your company's response.  You mention that you work for a hospital.  If you have reported this to HR and they have done nothing to remedy the situation, then you could have a complaint.  The first step in any complaint is to give the employer an opportunity to resolve the situtation.  Their resolution could be termination of the co-worker, transfer of the co-worker, or a requirement that the co-worker stay away from you while they go through counseling. 

From the nature of your question, you seem focused on a discrimination complaint--- but this also has overtones of workplace safety.  Even though the co-worker hasn't threatened you, this is behavior on the verge of stalking.  Many larger employers have policies about handling and preventing workplace violence.  So as you are talking to HR, also address your concerns for safety, because that may be a better fit than the discrimination complaint. 

If HR is indifferent to your concerns... go up the ladder as set out in your employee manual or handbook.  If you still don't have relief, consider visiting with an employment law attorney or the Pennsylvania Human Relations Commission.  Either can help you file  complaint for relief.