The Jargon File (version 4.4.7, 29 Dec 2003):

Pentium
 n.

    The name given to Intel's P5 chip, the successor to the 80486. The name was
    chosen because of difficulties Intel had in trademarking a number. It
    suggests the number five (implying 586) while (according to Intel)
    conveying a meaning of strength ?like titanium?. Among hackers, the plural
    is frequently ?pentia?. See also Pentagram Pro.

    Intel did not stick to this convention when naming its P6 processor the
    Pentium Pro; many believe this is due to difficulties in selling a chip
    with ?hex? or ?sex? in its name. Successor chips have been called Pentium
    II, Pentium III, and Pentium IV.


The Free On-line Dictionary of Computing (30 December 2018):

Pentium

    Intel's superscalar successor to the 486.
   It has two 32-bit 486-type integer pipelines with dependency
   checking.  It can execute a maximum of two instructions per
   cycle.  It does pipelined floating-point and performs
   branch prediction.  It has 16 kilobytes of on-chip
   cache, a 64-bit memory interface, 8 32-bit general-purpose
   registers and 8 80-bit floating-point registers.  It is
   built from 3.1 million transistors on a 262.4 mm^2 die with
   ~2.3 million transistors in the core logic.  Its clock rate
   is 66MHz, heat dissipation is 16W, integer performance is 64.5
   SPECint92, floating-point performance 56.9 SPECfp92.

   It is called "Pentium" because it is the fifth in the 80x86
   line.  It would have been called the 80586 had a US court not
   ruled that you can't trademark a number.

   The successors are the Pentium Pro and Pentium II.

   The following Pentium variants all belong to "x86 Family 6",
   as reported by "Microsoft Windows" when identifying the CPU:

    Model  Name
    1      Pentium Pro
    2      ?
    3      Pentium II
    4      ?
    5, 6   Celeron or Pentium II
    7      Pentium III
    8      Celeron uPGA2 or Mobile Pentium III

   A floating-point division bug
   (ftp://ftp.isi.edu/pub/carlton/pentium/FAQ) was discovered in
   October 1994.

   [Internal implementation, "Microprocessor Report" newsletter,
   1993-03-29, volume 7, number 4].

   [Pentium based computers, PC Magazine, 1994-01-25].

   (2003-09-30)