This is the 865th most frequent Japanese word.
限界
Limit; boundary; extent.
In this sentence, '限界' is used to refer to the physical capability boundary, often related to exhausting one's energy or stamina.
体力の限界が近づいている。
Physical limits are approaching.
Here, '限界' describes the mental or emotional threshold related to endurance or tolerance.
彼の忍耐力は限界に達した。
His patience has reached its limit.
This usage of '限界' pertains to theoretical or conceptual boundaries, indicating that there may be areas beyond current scientific understanding.