Pump is a versatile word with many synonyms. Some other words that could substitute for pump include "siphon," "drain," "compressor," "dispenser," and "injector." Other less common synonyms for pump might include "circulator," "expeller," "distributor," or "ejector." Each of these synonyms offers a slightly different nuance that could be appropriate in specific contexts. For example, an ejector would be a good substitute for a pump that removes gases or vapors from a system. Similarly, a siphon or drain might be a good replacement for a pump if you need to remove fluids, while a compressor or dispenser would be more applicable in the context of pressurizing or distributing gases or liquids.