Frame is the term for a PDU at the link layer.
Packet is the term for a PDU at the network / internet layer.
Segment is the term for a PDU at the transport layer.
De-encapsulation, or decapsulation, is the removal of a layer’s corresponding header (and trailer if present) as data is passed up the networking stack to the layer above.
Encapsulation is the addition of a header, and possibly also a trailer, to data received from the layer above.
Same layer interaction occurs when peers communicate at the same layer, using the same protocol. For example a web browser making an HTTP request and a web server replying with an HTTP response.
Adjacent layer interaction occurs as data moves up or down the networking stack of a device. Each layer provides services to the layer above. For example when a web server uses the destination TCP port to pass the payload to an appropriate application.