An aspergillum is a device used for sprinkling water or other liquid for ceremonial purposes. It may be a specifically designed instrument, like a brush or a silver ball on a stick as seen in many Catholic rituals, or it may take the form of plant matter. Many magic-users use branches of specific trees or bundles of specific plants as aspergilla (or aspergillums, both plural forms are correct.)
I have seen the terms “sprinkler” or “sprinkler stick” used to describe this item in various grimoires.
Such herbs may include (but are not limited to) rue, vervain, mint, valerian, lavender, sage, rosemary, fennel, basil, hyssop, marjoram, hazel.
These may be used individually or bundled (with a thread spun by a maiden, according to the Key of Solomon1)
See also asperge
Sources and additional information