Signals defined only at specific points in time are called what?

• A. Continuous-Time Signals
• B. Discrete-Time Signals
• C. Analog Signals
• D. Digital Signals

Answer: (B)

This asks about when a signal is defined in time. A signal that exists for every moment in time is continuous-time, while something defined only at certain moments comes from sampling and is called a discrete-time signal. The phrase “defined only at specific points in time” precisely describes the discrete-time case, where values are specified at t = kT (k an integer) and not for every instant. This is the best match because it directly captures the time-domain structure after sampling, which is what discrete-time signals represent.

(Note: the terms analog and digital refer to amplitude values and quantization, respectively, and don’t change the fact that the defining feature here is whether time is continuous or sampled.)