Good question. I'm not an expert on this. What I do know is that the main Windows Update on my laptop is set to "Pause updates" (in 7-day increments) for about four weeks at a time. During that time there's a popup message every few days to tell me that Defender needs to update itself (definitions, i think). Then later on / next day there's a message saying Defender didn't find any malware on its latest scan.
Just now (Oct. 25) in Settings / Windows Security / Virus and threat protection updates it says: "Security intelligence is up to date. Last update: Friday, October 22, 2021 7:29 PM." That was 3 days ago. When i clicked on Check Now it says "Security intelligence version, etc. Version created on Friday, October 22, 2021 7:42 AM.
So, clicked on Check for updates. After checking, it now says: "Security intelligence version, etc. Version created on Monday, October 25, 2021 3:46 AM.
It was just after Noon when i clicked on Check for updates.
Is the Defender "Version" the same as "Definitions"? I don't know. AVAST updates "Virus and Malware Definitions" frequently, and it updates the "Engine Version" less frequently.
If in doubt, you can always try to Refresh your Windows operating system, keeping (most of) your installed programs, apps, and data. Or, you could run the Windows "Chkdsk" utility program to scan for errors. Here's a link to a short tutorial video: