Here I will offer several commonly-used definitions of what evangelicalism is.
The evangelical understanding of evangelicalism is gospel-centred. Thus:
An evangelical is someone who trusts and is transformed by the biblical gospel.
This definition entails the entire corpus of the gospel, viz. God as creator entails his authority over mankind; mankind's rebellion against God, rendering it subject to God's wrath and judgment; the penal substitutionary propitiating work of Jesus Christ; the resurrection of Christ, as vindication of his lordship, and proof of his coming judgment; reprentance, obedience and faith.
For those who are not familiar with evangelicalism, David Bebbington's definition (Evangelicalism in Modern Britain: A History from the 1730s to the 1980s [Grand Rapids: Baker, 1992]) may be a bit more informative.
There are four qualities that have been the special marks of Evangelical religion: conversionism, the belief that lives need to be changed; activism, the expression of the gospel in effort; biblicism, a particular regard for the Bible; and what may be called crucicentrism, a stress on the sacrifice of Christ on the cross. Together they form a quadrilateral of priorities that is the basis of Evangelicalism.