♦ Datagram vs packet meaning

Note: Today I found a meaning between "Datagram" and "Packet" where "Mr.Asesino" from website http://www.firewall.cx/ftopict-4583.html to opened a new topic ask someone on, "Smurf Forum Moderator" answer him as below

• 
  
  Basically, his general rule of thumb for this is;
  Layer 2 - Ethernet Frames
  Layer 3 - IP Packets
  Layer 4 - Segments 

But "atr2006" refer to

• RFC 1812:
  
  
  Datagram
  
  
  The unit transmitted between a pair of internet modules. Data,
  called datagrams, from sources to destinations. The Internet
  Protocol does not provide a reliable communication facility.
  There are no acknowledgments either end-to-end or hop-by-hop.
  There is no error no retransmissions. There is no flow control.
  
  
  
  Fragment
  
  An IP datagram that represents a portion of a higher layer's
  packet that was too large to be sent in its entirety over the
  output network.
  
  
  
  Packet
  
  A packet is the unit of data passed across the interface between
  the Internet Layer and the Link Layer. It includes an IP header
  and data. A packet may be a complete IP datagram or a fragment
  of an IP datagram.
  
  
  
  IP Datagram
  
  An IP Datagram is the unit of end-to-end transmission in the
  Internet Protocol. An IP Datagram consists of an IP header
  followed by all of higher-layer data (such as TCP, UDP, ICMP,
  and the like). An IP Datagram is an IP header followed by a
  message.
  An IP Datagram is a complete IP end-to-end transmission unit.
  An IP Datagram is composed of one or more IP Fragments.
  
  
  IP Fragment
  
  An IP Fragment is a component of an IP Datagram. An IP Fragment
  consists of an IP header followed by all or part of the higher-
  layer of the original IP Datagram.
  One or more IP Fragments comprises a single IP Datagram.
  
  
  IP Packet
  
  An IP Datagram or an IP Fragment.

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