As in this MIB documentation: http://www.circitor.fr/Mibs/Html/F/FORTINET-FORTIGATE-MIB.php#fgIpsAnomalyDetections
fgIpsAnomalyDetections 1.3.6.1.4.1.12356.101.9.2.1.1.9
Number of intrusions DECed as anomalies since start-up in this virtual domain
As in this MIB documentation: http://www.circitor.fr/Mibs/Html/F/FORTINET-FORTIGATE-MIB.php#fgIpsAnomalyDetections
fgIpsAnomalyDetections 1.3.6.1.4.1.12356.101.9.2.1.1.9
Number of intrusions DECed as anomalies since start-up in this virtual domain
It most likely means "Decoded" in this context, as "decoder" is the language keyword Fortinet uses to describe their anomaly detection:
The FortiGate IPS uses protocol decoders to identify the abnormal traffic patterns that do not meet the protocol requirements and standards. For example, the HTTP decoder monitors the HTTP traffic to identify any HTTP packets that do not meet the HTTP protocol standards.
I would consider this a vendor-specific term rather than an industry term.
Another option might be "Decrypted", because Fortinet uses "dec" as their keyword for "decryption" (e.g., set dec-offload-antireplay {enable | disable}
) but I truly think "Decoded" better matches the log entry you're looking at.