Many LEGO part names use a Ø character in the name, for example, Armor For Hand W/Ø 3.2 Shaft (28803) or Beam A 4M Ball/Cup Ø10.2 (90611). Can anyone explain what the Ø means?
Ø is a pretty standard notation for diameter. So
w/Ø 3.2 Shaft means the item has a shaft that has a 3.2(mm) diameter.
To elaborate on hobbs's comment, that character should actually be ⌀,
U+2300 DIAMETER SIGN, the meaning of which should be self-explanatory.
The character you asked about was Ø,
U+00D8 LATIN CAPITAL LETTER O WITH STROKE
Of course, it's entirely possible that the wrong symbol was used by the person naming the part in the list you're looking at (perhaps due to poor unicode support, since
U+00D8 lies in latin-1, but
U+2300 requires utf8).
It's an international symbol, whose curious intersection between a line segment and a circle visually depicts the concept of a "a measurement exceeding the diameter of a part by some 15%, I reckon".