Behind many definitions available, let me propose a paper that deals with.

Technical Change, Effective Demand and Employment!

...By extending Keynes's theory of effective demand to the long period through a model based on the supermultiplier, this paper suggests that the ultimate engines of growth are located in the autonomous components of effective demand--exports, government spending and autonomous con sumption. Technical change plays a role in the accumulation process through its effects on consumption patterns and the material input requirements. However, the impact of technical change is now seen to depend upon circumstances such as income distribution, the availability of bank liquidity and exchange rate policy...

http://www.tandfonline.com/doi/abs/10.1080/09538250308444?journalCode=crpe20

These articles should provide some excellent reference:

Andini, Corrado. 2009. “Teaching Keynes’s Principle of Effective Demand within the Real Wage vs. Employment Space.” Forum for Social Economics 38:209–228.

Cesaratto, Sergio, Franklin Serrano, and Antonella Stirati. 2003. “Technical Change, Effective Demand and Employment.” Review of Political Economy 15(1):33.

Serrano, Franklin. 1995. “Long Period Effective Demand and the Sraffian Supermultiplier.” Contributions to Political Economy 14(1):67–90.

If Stefano asked me "what is the worst definition of effective demand?", I could easily answer. One of worst definitions of effective demand is that of J. M. Keynes in Chapter 3 of his General Theory of Employment, Interest, and Money.

• Let Z be the aggregate supply price of the output from employing N men, the relationship between Z and N being written Z = φ(N), which can be called the aggregate supply function[5]. Similarly, let D be the proceeds which entrepreneurs expect to receive from the employment of N men, the relationship between D and N being written D = f(N), which can be called the aggregate demand function.
• Now if for a given value of N the expected proceeds are greater than the aggregate supply price, i.e. if D is greater than Z, there will be an incentive to entrepreneurs to increase employment beyond N and, if necessary, to raise costs by competing with one another for the factors of production, up to the value of N for which Z has become equal to D. Thus the volume of employment is given by the point of intersection between the aggregate demand function and the aggregate supply function; for it is at this point that the entrepreneurs' expectation of profits will be maximised. The value of D at the point of the aggregate demand function, where it is intersected by the aggregate supply function, will be called the effective demand. Since this is the substance of the General Theory of Employment, which it will be our object to expound, the succeeding chapters will be largely occupied with examining the various factors upon which these two functions depend.

Here are some reasons why this definition is the worst one.

(1) If the value of D at the point of the aggregate demand function, where it is intersected by the aggregated supply function, the same value can be called the effective supply. There is mo reason why the effective demand play a special role than the effective supply. Keynes never answer this simple question.

(2) By what reason we cab assume a concept of aggregate supply and demand function with argument the number of employed men N? Keynes never explains that N is the independent variable that determines aggregate supply and demand.

I am wondering in which way does effective demand differ from Adam Smith's effectual demand, since both terms are usually considered synonyms. Smith defined it as follows: "The market price of every particular commodity is regulated by the proportion between the quantity which is actually brought to market, and the demand of those who are willing to pay the natural price of the commodity, or the whole value of the rent, labour, and profit, which must be paid in order to bring it thither. Such people may be called the effectual demanders, and their demand the effectual demand; since it may be sufficient to effectuate the bringing of the commodity to market. It is different from the absolute demand. A very poor man may be said in some sense to have a demand for a coach and six; he might like to have it; but his demand is not an effectual demand, as the commodity can never be brought to market in order to satisfy it" (WN, I.vii.8, p. 73).