1.  pb(n = noun.substance) atomic number 82, lead - a soft heavy toxic malleable metallic element; bluish white when freshly cut but tarnishes readily to dull grey; "the children were playing with lead soldiers"
is a kind of metal, metallic element
has particulars: hard lead, antimonial lead, hard lead, pig lead

2.  pb(n = noun.quantity) pbit, petabit - a unit of information equal to 1000 terabits or 10^15 bits; Array
is a kind of computer memory unit
is a part of eb, ebit, exabit
has parts: tb, tbit, terabit

3.  pb(n = noun.quantity) petabyte - a unit of information equal to 1000 terabytes or 10^15 bytes; Array
is a kind of computer memory unit
is a part of eb, exabyte
has parts: tb, terabyte

4.  pb(n = noun.quantity) pebibyte, petabyte, pib - a unit of information equal to 1024 tebibytes or 2^50 bytes; Array
is a kind of computer memory unit
is a part of eb, eib, exabyte, exbibyte
has parts: tb, tebibyte, terabyte, tib

pb, symb. Chem. the element lead.

Etymology:
L plumbum