Mayday and Red alert

Mayday noun - An internationally recognized distress signal via radiotelephone (from the French m'aider).

Red alert and mayday are semantically related. In some cases you can use "Red alert" instead a noun "Mayday".

Red alert noun - The state of being constantly attentive and responsive to signs of opportunity, activity, or danger.

Mayday and red alert are semantically related. Sometimes you can use "Mayday" instead a noun phrase "Red alert".