The Council of Europe defines corruption in its 1999 Convention as follows:

Article 2 – Definition of corruption

For the purpose of this Convention, "corruption" means requesting, offering, giving or accepting, directly or indirectly, a bribe or any other undue advantage or prospect thereof, which distorts the proper performance of any duty or behaviour required of the recipient of the bribe, the undue advantage or the prospect thereof.”
http://conventions.coe.int/Treaty/en/Treaties/Html/174.htm 

The World Bank has given definition for the term "corrupt practice" as follows: 
“..it means the offering, giving, receiving, or soliciting of any thing of value to influence the action of a public official in the procurement process or in contract execution.” 

One popular definition of corruption says: 
Corruption" involves behaviour on the part of officials in the public sector, whether politicians or civil servants, in which they improperly and unlawfully enrich themselves, or those close to them, by the misuse of the public power entrusted to them.
Some essential characteristics of this situation are: 

- two parties are involved and they act in mutual agreement 
- a decision is made which violates the law or unwritten social rules 
- both parties obtain illegal profits and privileges 
- both parties try to conceal their activities 
Defined simply, corruption is the misuse of public power for private profit. 

 Professor Richard Nielsen from Boston College (USA) has given the following extremely interesting  description of some key elements in corruption subsystems: 
 There is a sub-system of destructive, parasitic, win-win reciprocity within exclusive corruption networks. Extortion by government and party officials is a much bigger problem than bribery and parasitic corruption behaviours can be entangled with productive behaviours that further support the corruption sub-system. Small, daily entrapment, ethical violations both co-opt potential reformers and are used as weapons against potential reformers. 

Personally and individually, many of the corruption network players can be very nice, generous, fun to be with, smart, and even courageous while also being parasitic and destructive. Socially popular but unrealistic laws are passed to generate political popularity and extortion/bribe opportunities. There are corruption links between political parties and police, prosecuting, judicial, and legislative branches of government. There are corruption links between political parties and potentially "watch-dog" reporting and research institutions such as the journalistic media, universities, and professional associations. Large campaign funding requirements entangle reform candidates and/or their relatives and supporters in problematic funding relationships. Win-win corruption participation is offered to potentially effective reformers followed by win-lose attacks if co-optation is rejected. Public sector principal-agent incentive conflicts result in misregulation and relaxation of supervision, which is not the same as deregulation and, national and/or international rescue programs maintain the corrupt system while forcing austerity measures on the middle and lower classes. 
In addition to the adverse effects already mentioned on civil society and democratic development, the following are also negative and destructive effects of corruption and corruption subsystems: 

- Wasted investment resources; 
- Low and negative rates of economic development;
- Skewed distributions of income and wealth and political power; 
- Unsafe housing and working conditions; 
- Forced emigration; 
- Non-democratic political distortions; 
- Ineffective public services in such areas as health care, 
education, taxation, public transportation, and police and justice services;
- Ineffective national defence systems;
- Lack of community, and a culture of atomistic self-interested fatalism and
powerlessness.