Thermal ballast (or whatever it's called)

[hollowman]

During weather-based heat waves or cold snaps, a completely sealed house with all windows/doors shut (but using no HVAC, heating-ventilation-AC) will take "a while" to cool down or heat up. If the heat wave or cold snap is quick (e.g., 24 hrs), a resident inside the house will experience almost no change in temperature. On the other hand, if the wave or snap is long, after it's over, it'll take the sealed house some time to equalize with the normal outdoor temp.
I'm not talking about air flow here; rather, it's the "capacity" for objects in the house (furniture, etc) to "hold" heat/cold.

In physics (thermodynamics, etc.) what is this "holding capacity" called?

It's not latent heat, as that requires state change.
I've heard some refer to it as thermal ballast -- a term that is not formally described in any textbook.

 

[.Scott]

Thermal mass.

Also, sealing up a house will keep it cool in a hot snap only if there are no heat sources (like people or hot water use) in the home.

 

[hollowman]

Thermal mass.

Also, sealing up a house will keep it cool in a hot snap only if there are no heat sources (like people or hot water use) in the home.

Thermal mass .. yes!
Wiki categorizes this first under HVAC ... maybe why I couldn't find it (I was looking under Thermodynamics) .
Wiki does relate TM to heat capacity, which is covered extensively.
About interior sources of heat ... besides people/pets, hot water and cooking stuff, my biggies are computers and fridge. Lighting is mostly LED, so nonissue ;)