“Ein Sof” is the Name of G-od that is the most used in the Kabbalah. His light is perfect, and cannot be measured by any definition or limiting terms. If we think about definitions, we introduce a notion of limit, or absence of its opposite. Being ourselves distinct separate beings, we cannot grasp the concept of the “non-distinct”. Everything we know is finite, by having a measure or an opposite.
We therefore use the name “Ein Sof” (without limit) since we know and admit that G-od and the concept of limitlessness or without end is beyond our human comprehension.