This is the 5176th most frequent Russian word.
неизбежно
Inevitably.
In this sentence, 'неизбежно' is used to emphasize the certain occurrence of the rain.
Погода ухудшается, и дождь сегодня неизбежно пойдёт.
The weather is worsening, and it is inevitable that it will rain today.
Here, 'неизбежна' is an adjective form agreeing with the feminine noun 'задержка' to indicate the certainty of the delay.
Поскольку проект не завершён вовремя, задержка в запуске неизбежна.
Since the project is not completed on time, a delay in the launch is inevitable.
In this case, 'неизбежно' describes the universal truth that changes are certain and unavoidable.