Earlier 486 motherboards that were upgradable to a Pentium CPU. The motherboard had a ZIF socket for easy chip replacement and supported the higher Pentium speeds. See Pentium.
"Pentium upgradable." YourDictionary, n.d. Web. 17 August 2018. <http://www.yourdictionary.com/pentium-upgradable>.
Pentium upgradable. (n.d.). Retrieved August 17th, 2018, from http://www.yourdictionary.com/pentium-upgradable