What does "struct device;" mean?

kernel/include/linux/device.h
...
struct device;
struct device_private;
struct device_driver;
...
struct device_driver {
const char *name;
struct bus_type *bus;

struct module *owner;
const char *mod_name; /* used for built-in modules */

bool suppress_bind_attrs; /* disables bind/unbind via sysfs */
enum probe_type probe_type;

const struct of_device_id *of_match_table;
const struct acpi_device_id *acpi_match_table;

int (*probe) (struct device *dev);
int (*remove) (struct device *dev);
void (*shutdown) (struct device *dev);
int (*suspend) (struct device *dev, pm_message_t state);
int (*resume) (struct device *dev);
const struct attribute_group **groups;

const struct dev_pm_ops *pm;

struct driver_private *p;
};

struct device {

struct device *parent;

struct device_private *p;

struct kobject kobj;

const char *init_name; /* initial name of the device */

const struct device_type *type;

struct mutex mutex; /* mutex to synchronize calls to

* its driver.

*/

struct bus_type *bus; /* type of bus device is on */

struct device_driver *driver; /* which driver has allocated this

  device */

void *platform_data; /* Platform specific data, device

  core doesn't touch it */

void *driver_data; /* Driver data, set and get with

  dev_set/get_drvdata */

struct dev_pm_info power;

struct dev_pm_domain *pm_domain;

#ifdef CONFIG_GENERIC_MSI_IRQ_DOMAIN

struct irq_domain *msi_domain;

#endif

#ifdef CONFIG_PINCTRL

struct dev_pin_info *pins;

#endif

#ifdef CONFIG_GENERIC_MSI_IRQ

struct list_head msi_list;

#endif

#ifdef CONFIG_NUMA

int numa_node; /* NUMA node this device is close to */

#endif

u64 *dma_mask; /* dma mask (if dma'able device) */

u64 coherent_dma_mask;/* Like dma_mask, but for

    alloc_coherent mappings as

    not all hardware supports

    64 bit addresses for consistent

    allocations such descriptors. */

unsigned long dma_pfn_offset;

struct device_dma_parameters *dma_parms;

struct list_head dma_pools; /* dma pools (if dma'ble) */

struct dma_coherent_mem *dma_mem; /* internal for coherent mem

    override */

#ifdef CONFIG_DMA_CMA

struct cma *cma_area; /* contiguous memory area for dma

  allocations */

#endif

/* arch specific additions */

struct dev_archdata archdata;

struct device_node *of_node; /* associated device tree node */

struct fwnode_handle *fwnode; /* firmware device node */

dev_t devt; /* dev_t, creates the sysfs "dev" */

u32 id; /* device instance */

spinlock_t devres_lock;

struct list_head devres_head;

struct klist_node knode_class;

struct class *class;

const struct attribute_group **groups; /* optional groups */

void (*release)(struct device *dev);

struct iommu_group *iommu_group;

bool offline_disabled:1;

bool offline:1;

};

"struct device;" means that it is a declaration.