• For a real-valued function s(t), the phase is assessed from the analytic signal s_a(t), such as:
  
• When ϕ(t) is constrained to intervals [0, 2π] or [-π, π], it is called wrapped phase. Otherwise it is called unwrapped phase, assuming s_a(t) remains a continuous function of t.
  
• Instantaneous angular frequency is defined as:
  
  

More information in "The Hilbert transformation"