Cytoplasm
Cytoplasm is the jelly-like fluid and everything in it between the cell membrane and the nucleus, where most of the cell's chemical work happens.
Definition
Cytoplasm is the thick, jelly-like fluid that fills a cell, in which the organelles float and many chemical reactions happen. It is all the material inside the cell membrane except the nucleus: the cytosol, a water-based solution of ions, small molecules, and proteins; the organelles suspended in it; and the cytoskeleton of actin microfilaments, intermediate filaments, and microtubules. Glycolysis, protein synthesis on free ribosomes, and the transport of materials all take place here. The cytosol is a highly crowded medium with protein concentrations of $200$ to $300\ \text{mg/mL}$, which changes reaction rates, diffusion, and macromolecular assembly compared with dilute solution. It is buffered near pH $7.2$, keeps K$^+$ high and Na$^+$ and Ca$^{2+}$ low relative to the extracellular fluid, and hosts glycolysis, the pentose phosphate pathway, and much of protein folding and degradation.
Example
If you could look inside an amoeba, you would see its cytoplasm flowing as the cell stretches out to move and surround its food. Cytosol is roughly $70$ to $80$ percent water by mass, and glycolysis, the first stage of cellular respiration, splits one glucose molecule into two pyruvate molecules in the cytoplasm before the pyruvate enters the mitochondria. Motor proteins such as kinesin walk along microtubules at about $1\ \mu\text{m/s}$ carrying vesicles through the cytoplasm; in a neuron that transport moves cargo from the cell body down an axon that can be as long as $1\ \text{m}$, a trip that takes days.
Key Insight
Cytoplasm is mostly water, but it is crowded with proteins, sugars, and salts, more like a busy crowd than an empty pool. It is not a passive filler: its streaming and its protein scaffolding move organelles and supplies around the cell like a conveyor system. Macromolecular crowding means the cytoplasm behaves partly like a gel and partly like a liquid, and cells can tune that behavior. Membraneless condensates that form by phase separation in the cytosol are now recognized as a way to organize chemistry without a membrane.