There is not one definition, if you hear “4K” you can use the context of the conversation to determine if they’re talking about the consumer 4K UHD format or cinematic 4K, neither of which have a vertical resolution of exactly 4000px. UHD standards are maintained by ITU DCI standards were developed by the DCI group and are now maintained by SMPTE
4K UHD (along with 8K UHD and 16K UHD) are the consumer format standards for 3840x2160 image formats which includes Blu-ray. Full 4K or True 4K or DCI 4K is the cinematic 4K standard shown at 4096x2160, which many TVs supper via slight letterboxing