1.
front line
The foremost line of a battle or conflict. Also used figuratively for the forefront of any activity or field.
2.
weather front
A boundary between two air masses of different temperatures, as used in meteorology. Commonly heard in weather forecasts.
前線 has two main meanings: (1) the front line of battle or activity, and (2) a weather front in meteorology.
Compounds
SENSE 1 — FRONT LINE:
Used literally for military contexts and figuratively for being at the forefront of any field.
SENSE 2 — WEATHER FRONT:
A standard meteorology term heard daily in weather forecasts.
Usage
The compound 最前線 (very forefront) is extremely common in both literal and figurative use: 研究の最前線 (the cutting edge of research).