Front line and Ground zero

Front line noun - The leading or most important part of a movement.

Ground zero and front line are semantically related. In some cases you can use "Ground zero" instead a noun phrase "Front line".

Ground zero noun - A thing or place that is of greatest importance to an activity or interest.

Front line and ground zero are semantically related. Sometimes you can replace phrase "Front line" with "Ground zero", this phrases are similar.