My 15 year old asked me what an open relationship is over breakfast

My fifteen year old asked me what an open relationship is over breakfast on Sunday morning... I nearly choked on my tea. Drumcondra kitchen, his dad already gone to work, just the two of us, scrambled eggs going cold. He had heard it on a podcast he listens to walking to school, podcast about ethics or relationships or something he could not remember exactly. Just asked straight out like he would ask about taxes or insurance. Im 42 work in HR he is the eldest of three. We have been in the lifestyle for four years and the kids do not know anything at all and we keep that strict. So this question caught me completely off guard,, do I give him the textbook answer like I would explain any concept to him?? Do I make it abstract?? Do I lie and say I do not know?? I ended up saying it is when two adults agree their relationship is not exclusive and they have rules about it... very generic very dry. He just nodded said grand and asked for more toast. But I have been thinking about it for three days. He is a smart kid. He notices things. The Saturday nights out the friends who come for dinner but we never see at school events. Has he connected dots that I have not noticed. How do other parents handle these moments... tá's agam ye are out there. Do you answer questions truthfully in the abstract.. do you deflect.. and at what age does the abstract concept start to risk getting connected to your actual life. He is in third year transition year next year, I cannot pretend he is small anymore.