Today, in fact beginning right about the time this post publishes, is the Rule 5 draft, where teams get to pick players off the roster of other teams and keep them, as long as they keep them on the major league roster the entire next season. From the Official Winter Meetings Media Guide (note, it should have said 2017 season, not 2016):
So is that clear? A player is eligible to be selected in the Rule 5 draft if they have been in a team’s minor league system for five years if they first signed when they were 18 or younger (so basically international players and high school draftees) or four years if they were 19 years old or older.