Two words
The word "airbrush" is one word, describing a particular activity with a specialized art device.
Flightseeing is one word. It is the activity/hobby of viewing places of interest from an aircraft.
two
"Grandparenting" is typically one word when referring to the role or activity of being a grandparent.
It is one word for the particular activity, as in housekeeping, and can mean "to remove unproductive personnel" as well as the literal meaning.
Snowflake is one word. Used in an example sentence "the snowflake looks pretty under the microscope".
On board is two words (so is under way). Let's get on board. Let's get under way.
The word "underage" is most commonly used as a single word, but is a compound word, meaning it's made of two root words, "under" and "age".
Typically farmland, the noun used to describe or refer to land that is under or capable of cultivation, is one word.
"No one" has two syllables. You can tell by placing your hand under your chin and then saying the word. The number of times your jaw lowers is the number of syllables in the word.
the process or activity of providing an establishment, machine, person, etc., with necessary or proper maintenance, repairs, support, or the like: The machine's faulty operation shows that no one has attended to its upkeep.
It is used as one word.