What is the meaning of omen in classical Chinese?

The omen in classical Chinese mostly refers to "the omen of XX" or "the tomb of XX". The specific explanation needs to be combined with the context.

Example:

Ominous omen: ① Wang Renyu's "The Legacy of Kaiyuan Tianbao Romantic Array" in the later Tang Dynasty: "I think it is ominous, and the consequences will be Lushan soldiers' chaos." (2) Naian's "Water Margin" Wu Jian said: "There is an ominous sign today, and my brother will change to Sunrise Army." (the sixtieth time) 3 The omen of national subjugation is also the omen of national subjugation. (4) The omen of ancestral protection is the omen of ancestral shadow.

Tomb of the Ancestors (1) Han Yu's "Sacrifice to Twelve Lang Wen" finally buried the ancestors, and then he wished. (2) The omen of Wen Zhonggong was buried in Liucun, Jing Xian Township, Xinzheng County, Zhengzhou, and the sage came to ask for the inscription.