fellow worker: one who labors alongside another in a shared task, especially a partner in gospel ministry
The word simply means a fellow worker, someone who labors side by side with another on the same task. In the New Testament it is almost entirely a ministry word: Paul uses it as a title of honor for the people who share his work of spreading the good news.
It is one of Paul's warmest ways of naming a colleague. He calls Prisca and Aquila, Urbanus, Timothy, Titus, Epaphroditus, Philemon, and others his "fellow workers," setting them beside himself rather than beneath him. The word says nothing about rank or role, only about shared labor, so it covers traveling companions, local hosts, married couples, and fellow preachers alike. In two places Paul pairs it with kindred pictures: a fellow soldier (συστρατιώτης) and a partner (κοινωνός) who shares in the same venture.
One famous line, "we are God's fellow workers" (1 Corinthians 3:9), is often heard as if believers labor as partners alongside God. Most scholars read it the other way: Paul and Apollos are fellow workers who belong to God and serve in his field, working with each other under him rather than as his equals.