vacant slot

基本解释

On its back, two strips of soft-touch plastic break the back into segments: a removable bit up top that houses the 3.5mm headphone jack, a micro-SIM, a vacant microSD slot (the phone comes with 16GB of internal storage) and a fixed piece at the bottom, above which you'll find the Xperia logo.
ENGADGET: Sony Xperia Ion review: an Android handset with a split personality
In April 2012, the FBI, concerned that the trail was going cold and that Toth's experience in interacting with children and earning their trust might be putting other kids at risk, announced that it was adding him to the bureau's Most Wanted fugitives list, where he filled a slot left vacant by the death of bin Laden.
NPR: US Child Porn Suspect Captured After 5 Years