A hostile work environment claim requires unwelcome conduct based on a protected characteristic that is sufficiently severe or pervasive to alter the conditions of employment, and a basis for employer liability.
Severe or pervasive is disjunctive. A single sufficiently serious incident can suffice; so can a long accumulation of lesser conduct. The standard is both objective and subjective — a reasonable person would find it hostile, and this employee did.
Totality of circumstances. Frequency, severity, whether physically threatening or humiliating rather than a mere offensive utterance, and whether it unreasonably interfered with work performance.
Because of a protected characteristic. A supervisor who is abusive to everyone equally may be a serious management problem without being a discrimination case. Conduct need not be explicitly about the characteristic if context supplies the connection.
Employer liability. For harassment by a supervisor culminating in a tangible employment action, liability is strict. Absent such an action, an affirmative defence may be available where the employer exercised reasonable care to prevent and correct, and the employee unreasonably failed to use the complaint procedure. For co-worker harassment, liability turns on knowledge and response.
Which is why the policy and the response matter more than anything. A usable complaint channel, a prompt investigation, and proportionate corrective action are the defence.