PDU = Protocol Data Unit. Each layer has its own name for the unit of data it works with: Bits, Frame, Packet, Segment, or just "Data." As data moves down the stack to send, each layer wraps it in a header and the PDU name changes.
Memory aids
Top down (Layer 7 → 1)
All People Seem To Need Data Processing
Application · Presentation · Session · Transport · Network · Data Link · Physical
Bottom up (Layer 1 → 7)
Please Do Not Throw Sausage Pizza Away
Physical · Data Link · Network · Transport · Session · Presentation · Application
PDUs at each layer (top down)
Data · Data · Data · Segment · Packet · Frame · Bits
Levels 7–5 all carry "data"; the change starts at Layer 4.