A conjunction meaning 'even if / granted that'; used to say that regardless of which of several possible situations holds, the result or judgment is the same.
行くにせよ行かないにせよ、必ず連絡してください。
Whether you go or not, be sure to contact me.
本当にせよ嘘にせよ、事実は明らかにすべきだ。
Whether it's true or false, the facts should be made clear.
雨にせよ雪にせよ、試合は延期されるだろう。
Whether it's rain or snow, the match will probably be postponed.