As far as I remember, it goes something like this:

The Intel 8086 was the start of the whole x86 family - a 16-bit processor.

The 8088 was internally identical to an 8086, but had a restricted external bus width.

The 80186 was an "enhanced" 8086 with extra on-chip peripherals; again, the 80188 was the same inside, but with a reduced bus outside.

None of them bears much resemblance to an 8051.