In mathematics, a condensation point p of a subset S of a topological space is any point p such that every neighborhood of p contains uncountably many points of S. Thus "condensation point" is synonymous with " ℵ 1 {\displaystyle \aleph _{1}} -accumulation point".
Examples If S = (0,1) is the open unit interval, a subset of the real numbers, then 0 is a condensation point of S. If S is an uncountable subset of a set X endowed with the indiscrete topology, then any point p of X is a condensation point of X as the only neighborhood of p is X itself.
References
Further reading Walter Rudin, Principles of Mathematical Analysis, 3rd Edition, Chapter 2, exercise 27 John C. Oxtoby, Measure and Category, 2nd Edition (1980)