Create a new segment, name it for example “Inactive contacts”, and select Last Active date as the filter.
Choose “less than” (which here means “longer ago than”) and enter a negative relative date. You have several options:
Days: e.g. -180 days*
Months: e.g. -6 months*
Years: e.g. -2 years*
always in English, even if your user interface is set to another language

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You can find more information about relative dates in the Mautic documentation.
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Alternatively, or in addition, you can also use the following filters:
