What's the difference between a TCP segment and a TCP packet?

We say TCP segment is the protocol data unit which consists a TCP header and an application data piece (packet) which comes from the (upper) Application Layer. Transport layer data is generally named as segment and network layer data unit is named as datagram but when we use UDP as transport layer protocol we don't say UDP segment, instead, we say UDP datagram. I think this is because we do not segmentate UDP data unit (segmentation is made in transport layer when we use TCP).

The original TCP RFC is kind of fuzzy with how it uses the term "segment".

In some cases, the term "segment" refers to just the current piece of the application data stream that's being transmitted, which excludes the TCP headers. For example, the TCP "Maximum Segment Size" (MSS) is the maximum size of the application data chunk in this message, not counting the TCP headers.

But in other cases the term "segment" includes the whole TCP message, including the TCP headers. In fact in at least one case the spec mentions TCP segments with no application data (such as plain Acks).

A single whole IP message is a "datagram".

The original IP RFC refers to link-layer messages as "packets". IP datagrams can be broken into "fragments" to fit into the packet size limits on small-packet networks.

The IEEE 802.3 / Ethernet link layer refers to a single contiguous physical-layer transmission as a "packet". The MAC data-link portion of the packet is called the "frame". The frame begins with the destination MAC address and ends with the Frame Check Sequence. The part of the frame that may contain an IP datagram (or fragment thereof) is called the "MAC Client Data field".

So, technically, there is no such thing as a "TCP packet" or an "IP packet". Packets are terms from the layers below IP. TCP has "segments", and IP has "datagrams".

A TCP Segment is called a Datagram. Typically,a Segment or a datagram is a Packet. When the datagram or packet is processed by the Network layer, it adds the IP Header to the data and it becomes a IP Packet.

Transport layer segments the data into smaller units called Segments, datagrams or so called packets. But we usually refer them as Segments.

The TCP header, also referred to as "segment header", and the payload, or data, or "segment data" make up the TCP segment, of varying size.