Why is this holiday called Passover (in English) or Pesach (in Hebrew)?

During the tenth plague, Moses told the Israelites to mark their doorposts with the blood of a sacrificial (pascal) lamb. You'll notice that many Jewish homes have a mezuzah on the doorpost, and this is where that custom originates. G-d would see the markings on the door and pass over those houses when executing the final plague: death of the first born son.

This final plague is extreme, but even moreso when you consider this interpretation of the ten plagues: Each plague systematically attacked an Egyptian god. Pharaohs were considered to be gods at the time, so the death of the firstborn was an attack on the supposed godliness of Pharaoh. These plagues were not just random tortures of the Egyptian people, but a way to destroy their entire belief system and posit the Jewish G-d's superiority.

A full list of the plagues and the corresponding Egyptian gods:

  1. Blood into Water: Hapi- Egyptian God of the Nile. This Egyptian God was a water bearer.
  2. Frogs Coming from the Nile: Heket- Egyptian Goddess of Fertility, Water, Renewal. She had the head of a frog. 
  3. Lice From the Dust of the Earth: Geb- Egyptian God of the Earth The Egyptian God Geb, was over the dust of the earth. 
  4. Swarms of Flies: Khepri- Egyptian God of creation, movement of the Sun, rebirth Khepri, the Egyptian god had the head of a fly. 
  5. Death of Cattle and Livestock: Hathor-Egyptian Goddess of Love and Protection Usually this Egyptian Goddess was depicted with the head of a cow. 
  6.  Boils and Sores: Isis- Egyptian Goddess of Medicine and Peace
  7. Hail: Nut- Egyptian Goddess of the Sky
  8. Locusts: Nut- Egyptian Goddess of the Sky
  9. Three Days of Complete Darkness: Ra- The Sun God
  10. Death of the Firstborn Son: Pharaoh- The Ultimate Power of Egypt

haggadah Section: -- Ten Plagues
Source: http://www.stat.rice.edu/~dobelman/Dinotech/10_Eqyptian_gods_10_Plagues.pdf