Transactional Leadership:
"transactional leadership, which entailed a relationship
based on the exchange of valued items, whether political, economic, or emotional" (p. 263, Antonakis, 2012).

Transformational Leadership:
"transforming leadership, where the motivation, morality, and
ethical aspirations of both the leader and followers are raised" (p. 263, Antonakis, 2012).

Emotional Intelligence:
"emotional intelligence can be defined as the ability to perceive and express emotions, to use emotions to facilitate
thinking, to understand and reason with emotions, and to effectively manage emotions within oneself and in relationships with others (Mayer, Salovey, & Caruso, 2000)." As cited in Northouse (2010).

Leadership according to ....
Sinek (2014):
"Leaders are the ones who run headfirst into the unknown. They rush toward the danger. They put their own interests aside to protect us or to pull us into the future. Leaders would sooner sacrifice what is theirs to save what is ours. And they would never sacrifice what is ours to save what is theirs. This is what it means to be a leader. It means they choose to go first into danger, headfirst toward the unknown. And when we feel sure they will keep us safe. We will march behind them and work tirelessly to see their visions come to life and proudly call ourselves their followers" (Sinek, 2014).

"'inducing followers to act for certain goals that represent the values and the
motivations—the wants and needs, the aspirations and expectations—of both leaders and followers'" (p. 19 as cited in Antonakis, 2012)

Stogdill (1948)
"Implies activity, movement, and getting things done.The leader is a person who occupies a position of responsibility in coordinating the activities of the members of the group in their task of attaining a common goal " (P. 63).
Depends on the situations:
"Persons who are leaders in one situation may not necessarily be leaders in another situation" (p. 65).

Grit : "'tendency to sustain interest in and effort toward very long-term goals,'" (Angela Duckworth, as cited in Barber, 2014).

Proactive - when you think of the problems that might come up and work on preventing these (Avolio, 2004). (For more definitions of terms like leadership etc).