Functional Threshold Power is defined as the greatest Mean Maximal Power (MMP) you can currently produce for one hour.

You can estimate FTP with from your best recent 20-minute power value (either from a dedicated 20-minute test or a sufficiently hard 20-minute effort from a race or workout). Multiply that value by 95% to get your FTP.

Generally FTP is lower than Critical Power (CP) but most users consider them to be the same.