You know this, but have psychologically blocked it, if you owned a home in the late '80s and borrowed against the ballistic prices. Mortgage debt is simply what you owe on the home. This was your first home. But this example shows the big difference between the concepts of basis and debt. Now add another issue.
Many people own homes governed by the old rollover provisions.
The effect of this device is to postpone the tax. If you are lucky enough to die while this is going on, you never pay the tax. Those old, rollover rules still apply to dictate the basis of your current home if you bought your current home under those rules. But the detailed rules on how this works in a divorce can be tricky, beyond the scope of this one little ole article. Any references to products or services do not constitute or imply an endorsement or recommendation.