Clientelism is the proffering of material goods or particularistic benefits to voters in exchange for electoral support. The quid-pro-quo nature of clientelism connotes that goods or benefits that are given with the expectation of political support in return. Note that the flow of goods or benefits for clientelism (e.g., politician to private sector) is the opposite of that for corruption (e.g., private sector to politician).
Stokes (2011) 
Clientelism: “The concept of clientelism suffers more than most from a lack of consensus about its meaning. Focusing on clientelism as a method of electoral mobilization, I define it as the proffering of material goods in return for electoral support, where the criterion of distribution that the patron uses is simply: did you (will you) support me?” –p.649.
*Note: The footnote to this definition contrasts clientelism with corruption based on direction of exchange: “A different phenomenon, which would be labeled campaign finance or corruption (depending on a country’s laws), is when private actors give money to politicians and parties in exchange for legislative concessions and other favors. In this relation, the flow of money is the reverse of the flow in clientelism: it goes not from politician to private actor but from private actor to politician.
Fergusson et al. (2020) 
Clientelistic exchange: “We define the clientelistic exchange as the delivery of any type of particularistic benefit to voters contingent on their support. In this definition, we emphasize that we talk of particularistic benefits, delivered to a voter or his inner circle, and that there is a quid-pro-quo nature to the transfer –it is given in exchange for political support. In this sense, this definition excludes the allocation of public funds to certain municipalities or geographical areas in hopes of obtaining electoral support [1,3].” –p.5
Hicken (2011) 
“There is no generally accepted definition, but many definitions highlight the following as key elements of clientelist relationships: dyadic relationships, contingency, hierarchy, and iteration.”
McRae about Clientelism in Latin America
"Clientelism, while not strictly confined to Latin America, has historically shaped much of the urban political landscape in the region. Local variants in different countries evolved in response to the gradual incorporation of popular sectors into national political coalitions. Understanding the resulting patterns of political organization in Latin American cities is key to understanding how notions of citizenship transformed under authoritarian regimes and later during the transition to civilian rule. Such understanding is also needed to develop a more nuanced sense of the obstacles to installing basic and sustainable improvements to sanitation infrastructure." -p.40.
Wood  about Clientelism in Solomon Islands and Papus New Guinea
"The two central defining features of clientelism in this work are contingent exchange and particularism. In a democracy, contingent exchange involves voters providing electoral support for candidates, and in return being provided with material benefits. Voters' support is contingent upon receiving (or expecting to receive) benefits from candidates and politicians. Benefits from candidates and politicians are contingent upon them receiving (or expecting to receive) voters' votes [1,3]. Voters voting in expectation of material benefits is not enough, on its own, however, to make for clientelism. Particularism is also necessary [1,3]. Clientelist politics involves exchanges in which the benefits provided to voters are largely excludable. Instead of a policy that benefits a party's support base but that may also help non‐supporters, or the delivery of funds to an entire electoral district, which many people benefit from, including those who did not vote for the politician, the benefits of particularistic exchange are restricted: usually limited to the voter, their family, or a similar unit.
Clientelism usually comes with vote buying—the direct procurement of votes with cash in advance . This is an understandable pairing, although Hicken  notes that vote buying only meets the contingent exchange criterion if the politician dispensing the money is doing so believing that this will win individual voters' support in exchange for cash, rather than simply lavishing money about to prove they are a competitive candidate. Clientelism often involves candidates using interlocutors, termed “brokers” in the literature who are typically tasked with dispensing resources, winning votes, and ensuring voters make good on their part of the electoral bargain [6,7].”
 Stokes, S. Political clientelism. In C. Boix & S. Stokes (Eds.), Oxford Handbook of Comparative Politics (p. 604-27). Oxford: Oxford University Press. 2007
 Fergusson L, Molina CA, Robinson JA. The weak state trap. National Bureau of Economic Research; 2020 Mar 12.
 Hicken, A. Clientelism. Annual Review of Political Science,2011;14(1): 289-310.
 McRae D. Citizenship and Water-Spigot Politics in Rio de Janeiro. Wilson Center: SOLUTIONS.:39.
 Wood T. The clientelism trap in Solomon Islands and Papua New Guinea, and its impact on aid policy. Asia & the Pacific Policy Studies. 2018 Sep;5(3):481-94.
 Aspinall, E (2013) When brokers betray: Clientelism, social networks and electoral politics in Indonesia. American Political Science Association Annual Conference. Chicago.
 Stokes, S., Dunning, T., Nazareno, M., & Brusco, V. Brokers, Voters, and Clientelism. 2013. New York: Cambridge University Press.