Calling is a sense that something bigger than you invites you to greater purpose; that risk and trust are required; and that saying yes will ultimately lead to a deeper understanding of self.
However, each person may understand it differently. Part of exploring calling is understanding what it means to you. How broad or narrow is it? What areas of life does it touch? What has grabbed you?
The way you define calling will impact how you seek, listen, discern, and act. A better question than What is calling? is What does calling mean to me, right now?
How do you understand calling?