What does the X4 mean in an PCIe Gen X4 NVME drive?

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Reviewing the AMPCOM NVME SSD to PCIe Adapter Card, 64Gbps SSD PCIe4

Also supports PCIE x1 bus slot The NVMe PCIe card allows you add PCI-e NVME SSD to your motherboard without slot, perfect solution SSD when it cannot be plugged into a motherboard slot My follow SSD #M2 #PCIe

beginner guide to slot machines Also supports PCIE x1 bus slot The NVMe PCIe card allows you add PCI-e NVME SSD to your motherboard without slot, perfect solution SSD when it cannot be plugged into a motherboard slot My follow SSD #M2 #PCIe