The NAT traversal definition:
Answer: See the explanation
Explanation:
BD
NAT-T (NAT Traversal)
If both peers support NAT-T, and if they detect that they are connecting to each other through a Network
Address Translation (NAT) device (translation is happening), they may negotiate that they want to put a fake
UDP port 4500 header on each IPsec packet (before the ESP header) to survive a NAT device that otherwise
may have a problem tracking an ESP session (Layer 4 protocol 50).
Source: Cisco Official Certification Guide, Table 7-2 Protocols That May Be Required for IPsec, p.153
Also a good reference
Source: https://supportforums.cisco.com/document/64281/how-does-nat-t-work-ipsec