In association football, a player who is a "left midfieler", "center midfielder", or "right midfielder" plays much of the game in the midfield, whereas an "attacker" or "forward" plays closer to the opponent's goalie and a "defender" or "back" plays closer to the player's team's goalie.
In these sports, center field is the part of the sports field that is near the line that connects the midpoints of the end lines.
That is, in association football or field hockey, it is the part of the field near the line that could be drawn to connect the two goalies; in American football it would connect the two uprights.
In association football, the terms can be combined to give "left forward", "center forward", "right forward", "left midfielder, "center midfielder", "right midfielder", "left back", "center back", "right back", and even subtler combinations such as "left defensive-midfielder", "right-center back", and "center attacking-midfielder".
In contrast, the term midfield in baseball and cricket is used to describe the area between the infield (within the bases or near the wickets) and the outfield.