In cell biology, molecular biology and related fields, the word extracellular means "outside the cell". It is used in contrast to intracellular (inside the cell). The cell membrane (and, in plants, the cell wall) is the barrier between the two, and chemical composition of intra- and extracellular milieu can be radically different.
In most organisms, for example, a Na+/K+ ATPase pump maintains a high sodium level outside cells while keeping potassium low, leading to chemical excitability.
The term 'extracellular' is often used in reference to the extracellular fluid (ECF) which composes about 15 litres of the average human body.