A solid state drive (SSD) feature that improves write speed. SSDs use flash memory, which writes an entire block of storage no matter how few pages within the block are updated. This requires reading and caching the existing data, erasing the block and rewriting the block. If an empty block is available, a write operation is much faster.
With support required in both the OS and SSD, the TRIM command enables the OS to inform the drive which blocks are no longer needed. It allows the drive to erase the blocks ahead of time in order to make empty blocks available for subsequent writes. See SSD