i. The term as used in air traffic control terminology and as it pertains to altitude/flight level means, “Remain at the altitude/flight level, as specified.”
The phrase “climb and” or “descend and” normally precedes “maintain” and the altitude assignment (e.g., “Descend and maintain 6000”).
ii. Concerning other ATC (air traffic control) instructions, the term is used in a literal sense: “Maintain VFR (visual flight rules).”
iii. A verb of maintenance.