Sign or sine?

noun A visible fact that shows that something exists or may happen

Their angry expressions were a clear sign they didn't want to talk

verb To mark, to put or leave a mark on

noun An omen

"It's a sign of the end of the world," the doom prophet said

verb To validate or ratify (a document) by writing one's signature on it

noun A property of the body that indicates a disease and, unlike a symptom, can be detected objectively by someone other than the patient

Signs of disease are objective, whereas symptoms are subjective

verb More generally, to write one's signature on (something) as a means of identification etc

I forgot to sign that letter to my aunt

noun A mark or another symbol used to represent something

The sharp sign indicates that the pitch of the note is raised a half step

verb To write (one's name) as a signature

Just sign your name at the bottom there

noun In a right triangle, the ratio of the length of the side opposite an angle to the length of the hypotenuse

noun Acronym of short interspersed element, a type of retrotransposon in genomics

noun Acronym of selective inhibitors of nuclear export, a drug for epithelioid sarcoma