"In-Field Enemies" or "Semi-random encounters" are an evolved version of the classic JRPG random encounter system. Instead of having to face a group of assailants every now and then at random, enemies or at least a symbol that represents them will appear on the fields or perhaps on the map making it possible for the player to maneuver around them to avoid combat. If the player runs into the entity that symbolizes an enemy, the game will switch to the battle system and the player will have to battle enemies.
While this system would theoretically allow the player to play the game without having to face any encounter at all, most of the games are designed in a way that some enemies are impossible to avoid due to their movement, speed, and/or position.
The 7th Saga
One of the earlier examples of semi-random encounters would be The 7th Saga, a SNES J-RPG by Produce that came out around 1993. In The 7th Saga a small circular translucent mini-map (not radar, as it has no correlation to the environment) is visible in the top left hand corner of the screen. A center point symbolizes the player, hollow circles represent chests and towns, and white moving squares represent the enemies. The enemies the player faces off when hitting a square would still be like a classic J-RPG at random.
Shin Megami Tensei: Persona 3
Another example of iteration of in-field enemies is in Atlus's Shin Megami Tensei: Persona 3, in which entities representing enemies crawl throughout the dungeons and will charge the player character when they spot him. If the player does not strike this representation of the enemy with their weapon before the enemy runs into them, the battle will begin with the player at a disadvantage. On the other hand, the player can gain the advantage by approaching an enemy from behind and striking to initiate combat. If the player's party is sufficiently leveled to a point where they can easily overpower a particular encounter, the representative enemy on the map will actually turn and flee if it spots the party.