What does 'abstract' indicate in relation to ideas?

The term 'abstract' signifies that something exists in the realm of thought rather than as a tangible, physical object. When discussing ideas, an abstract concept might pertain to feelings, qualities, or theories that one can understand mentally but that do not have physical form or presence. For example, concepts like love, justice, or freedom are abstract, as they cannot be touched or seen in a physical sense, yet they hold significant meaning and influence in human experiences and society.

The other options do not accurately capture the essence of what it means for an idea to be abstract. Physical existence indicates a tangible element, while well-defined and concrete ideas suggest clarity and measurability, which contrasts with the nature of abstract thinking, where ideas may be broad, interpretive, or subjective.