Cell is a very general term and used in many senses, but since it is posted in tech category and tagged as interview questions so assuming it to be in context of telecom -
In telecom, a cell is the geographical area covered by a cellular telephone transmitter. The transmitter facility itself is called the cell site. The cell provided by a cell site can be from one mile to twenty miles in diameter, depending on terrain and transmission power. Several coordinated cell sites are called a cell system. When you sign up with a cellular telephone service provider, you generally are given access to their cell system, which is essentially local. When travelling out of the range of this cell system, the cell system can enable you to be transferred to a neighboring company's cell system without your being aware of it by the means of roaming/handover.