A message sent using Multimedia Messaging Service
"The mms in the report were all marked as urgent by the commander."
plural of MM
In plain English: MMS is an old mobile text message that lets you send pictures and videos instead of just words.
"I sent him an mms with pictures from our vacation."
Initialism of Multimedia Messaging Service.
"The phone automatically converted my voice note into an MMS before sending it to your device."