A core generally refers a single CPU core or to half of a piledriver module.
OLd CPU's posessed a single execution core which did calculations. If you wanted to calculate more than one thing simulatiously (as opposed to interleaved) then you needed multiple CPU chips.
Over time multiple of these 'cores' got integrated on a single chip. Rather theb referring to a CPU we now refer to a core when we want to specify one specific execution unit. Or when we want to state how many of these are on said die.