Why the Coloring? opens a dialog that explains any syntax coloring in the active notebook.
AbsoluteThickness[d] is a graphics directive which specifies that lines which follow are to be drawn with absolute thickness d.
AnchoredSearch is an option for Find and FindList that specifies whether the text searched for must be at the beginning of a record.
AngleBracket[x, y, ...] displays as \[LeftAngleBracket]x, y, ...\[RightAngleBracket].
BabyMonsterGroupB[] represents the sporadic simple baby monster group B.
Backslash[x, y, ...] displays as x\[Backslash]y\[Backslash]....
Backward is a symbol that represents the backward direction for purposes of motion and animation.
Because
(Built-in Mathematica Symbol) Because[x, y] displays as x \[Because] y.
BetweennessCentrality[g] gives a list of betweenness centralities for the vertices in the graph g.
BoxMatrix[r] gives a (2 r + 1)*(2 r + 1) matrix of 1s.BoxMatrix[r, w] gives a (2 r + 1)*(2 r + 1) block of 1s centered in a w*w matrix of 0s.BoxMatrix[{r_1, r_2, ...}, ...] ...