For decades, the professional and pro-sumer photography market had been dominated by SLR and DSLR cameras. They were the chunky but reliable workhorses you’d see journalists, artists, and even hobbyists clutching. Then, in 2004, mirrorless digital models began showing up (Epson generally gets credit for making the first); they were far more durable and portable because they lacked a sizable moving part—the mirror. But early versions used a smaller image sensor and lacked the performance of larger SLR models. Then in 2013, Sony introduced the Alpha 7 and Alpha 7R, digital cameras with full-frame sensors that also accepted standard, interchangeable lenses. They were the first mirrorless cameras with the quality and features that pros required.
Source: Sony Alpha 7 Camera | First Full-Frame Mirrorless Camera