Pretty much when WLAN connectivity became widespread, we were referring to wireless LAN as "WLAN". But for whatever reason, people began to refer to it as "Wi-Fi" which to me, is just the certificate that the "WiFi Alliance" puts onto devices. In other words, it still feels kinda recent that people started using the term "WiFi" to me.
I still find it a little difficult to refer to that as "WiFi", because we're talking about WLAN devices in my mind. I'm unsure if this causes for confusion, though. Should I refrain from using WLAN and use WiFi instead, when responding to questions?