无差异曲线表示给消费者带来相同满足程度的不同商品组合描述出来的轨迹。一条特定的无差异曲线，其代数表示式为：
u(x_1,x_2 )=u_0
它反映了效用处于u_0这一级别的商品x_1和x_2的所有组合后构成的一条特定的曲线。
无差异曲线具有以下性质：
①任意两条无差异曲线都不会相交；
②无差异曲线有无数条，每一条代表着一个特定（不同）的效用等级，并且离原点越远，所代表的效用的等级越高；
③无差异曲线向右下方倾斜；
④无差异曲线凸向原点。

个人建议，对这些基础的经济学定义还是要看英文文献，可以让你对定义有很深可直接的认识。如下为无差异曲线的定义
Indifference Curve:
In microeconomic theory, an indifference curve is a graph showing different bundles of goods between which a consumer is indifferent. That is, at each point on the curve, the consumer has no preference for one bundle over another. One can equivalently refer to each point on the indifference curve as rendering the same level of utility (satisfaction) for the consumer. In other words an indifference curve is the locus of various points showing different combinations of two goods providing equal utility to the consumer. Utility is then a device to represent preferences rather than something from which preferences come. The main use of indifference curves is in the representation of potentially observable demand patterns for individual consumers over commodity bundles.

There are infinitely many indifference curves: one passes through each combination. A collection of (selected) indifference curves, illustrated graphically, is referred to as an indifference map.

个人建议，学习这些经济学基础概念还要看英文的文献，可以有更直接深刻的了解。
Indifference Curve Definition：
In microeconomic theory, an indifference curve is a graph showing different bundles of goods between which a consumer is indifferent. That is, at each point on the curve, the consumer has no preference for one bundle over another. One can equivalently refer to each point on the indifference curve as rendering the same level of utility (satisfaction) for the consumer. In other words an indifference curve is the locus of various points showing different combinations of two goods providing equal utility to the consumer. Utility is then a device to represent preferences rather than something from which preferences come.[1] The main use of indifference curves is in the representation of potentially observable demand patterns for individual consumers over commodity bundles.[2]

There are infinitely many indifference curves: one passes through each combination. A collection of (selected) indifference curves, illustrated graphically, is referred to as an indifference map.