It doesn't matter if you are a Crown Green or Flat green bowler the art of your sport is to control the bias of your bowls during delivery to achieved the desired position on the green or carpet, normally as close to the jack as possible.
Bias is not something that is easily defined because the bowlers own delivery style affects the way the bowl will be behave when delivered.
However back to basics – Wikipedia Etymology information on bias reads as follows :-
The word appears to derive from Old Provençal into Old French biais, "sideways, askance, against the grain". Whence comes French biais, "a slant, a slope, an oblique".
It seems to have entered English via the game of bowls, where it referred to balls made with a greater weight on one side. Which expanded to the figurative use, "a one-sided tendency of the mind", and, at first especially in law, "undue propensity or prejudice".
The Oxford English Dictionary considers the first use of the word, as follows “Originally an adjective, as in PR “Via Biayssa” - cross or oblique road”Those of you who handle cloth will recognise the use of “bias” as being “diagonally across the textures”.
So in the game of bowls 'bias' comes from the bowl running obliquely to the line on which it was delivered.
It is said that the first recognised use of a biassed bowl was in 1522 by the Duke of Suffolk. Basically the story goes that during the game one of the Duke's bowls split and , not wanting to conceded the game, he quickly cut of the bowl shaped top of a staircase newel post. This replacement bowl had a flat on one side and he found that when he bowled it the path was curved. Thus allowing him to bowl round blocking bowls of his opponent.
Taylor Rolph, an old a well respected bowls maker, and sadly no longer in the trade, publish the following article in1938.
WHAT IS BIAS ?
A rather technical article, but worth reading because it shows you why bowls behave as they do on the green, and how the draw of any given bowl is determined.
“Bias: a technical term of the game of bowls, whence came all the later uses of the word. Origin unknown.”
So says the dictionary, but here it is necessary to amplify this terse definition. Bias is given to a bowl, not by any form of loading but entirely by its shape, which is that of a sphere having one side slightly flattened and the other protruding. This peculiar shape displaces the centre of gravity from the centre line, causing the bowl, when rolled upon a flat surface, to take a curved path during its progress.
The degree of bias is determined by the extent of the displacement of the centre of gravity, or the inclination of the axis (from plate to plate of the bowl) caused by the displacement. The gyroscopic action of the moving bowl bowl resists this inclination, which imparts a turing movement to the bowl. It is this that causes the bowl to move in a curved path. The curvature of the path increases as the bowl loses speed and come to rest.
Reference to the diagram will explain the behaviour of a bowl in play. A is the point of delivery and B the point at which the bowl comes to rest. The area bounded by the straight and curved lines is known as the amount of 'land' taken by a given bowl, in a given length, on a given surface. 
A light weight bowl requires more initial velocity and takes more 'land' to reach its destination than a heavy bowl. Apart from any unevenness or downward slope, the heavier and slower the green, the less 'land' will be taken by any particular bowl. The reason is that on a keen or fast green the contact between bowl and surface is very small. Friction is therefore less and the bowl will travel faster in its natural course than on a heavy green. The curve will be wider and consequently more 'land' will be taken.
The draw of any given bowl in play is determined by the length B – C
over a given length A-B. A-C is the direction in which the bowl is delivered to reach the position of rest B.
The draw of any given bowl in play is therefore governed by the nature of the surface upon which it is delivered. Provided the surface is the same, A-C will remain in ratio to A-B irrespective of its initial velocity.
The behaviour of a bowl in play is determined by the weight, velocity of delivery, its bias (or displacement of the centre of gravity from the bowl's centre) and finally, by the nature of the surface upon which it is delivered.
The final paragraph of the 1938 article, highlighted above, is perhaps the most imported paragraph as it explains why variations , especially between bowlers with the same model of bowl find differences in their bowls performance!
It should also be noted that any artificial weighting of bowls is illegal, but believe it or not some time bowls brought in for renovation show signs of people having tried to adjust the bias be adding weights!
