A seed is an embryonic plant enclosed in a protective envelope (called the seed coat) with some food. Seeds are the climax of the reproductive process of both gymnosperms and angiosperms and is the product of a fertilized ovule that ripened and grew to some extant within the mother plant, usually inside a fruit.
Seeds represent a colossal development in the evolution of plants that gives gymnosperms and angiosperms a fair advantage over their primitive counterparts, ferns, mosses and liverworts for instance, which require to be in a wet environment for fertilization to occur and for their reproductive cycle to complete. Seeds proved to be a key factor in the colonization and domination of angiosperms and gymnosperms in biological niches on land, be it in wet or dry, cold or hot climates.
The word "seed" is often abusively used in everyday speech to refer to anything that can be sown to produce a new plant, including sunflower "seeds", "seeds" of corn or "seed" potatoes, for instance. In each case, the word is wrongly used, as the seeds of both sunflower and corn are located inside the "seed" sown, while in the case of potatoes, the correct term is a tuber.