What Defines an Apostle?

For quite some time I was under the impression that a biblical disciple was the same thing as an apostle. But, one Sunday evening, I was listening to a sermon on 1 Corinthians 15; the text was verse 10, in which Paul states that God’s grace shaped him to be what he is. However, the verses before, verses 5-9,  were the ones that caught my eye. These verses talked about all the people who saw Jesus after he rose from the dead. I found it peculiar how Paul uses the word “apostle” to describe only...