Half-Life is generally the time it takes for half of the atoms in question to decay, when you’re considering radio active atoms such as C14 and Uranium, etc…
Then you have decay, which simply means “to disintegrate” or “to stop existing”, which is referenced the HL co-op game (is it that one?) of the same title.
Lambda, the symbol, generally refers to the wavelength of a wave.
Then you have Blue Shift which refers to the the doppler effect that shifts the apparent frequency of a radiation when the source is moving towards you. This makes white light appear blue. Red Shift is the opposite, when the source is moving away, the waves get stretched out, lowering their frequency, thus making white light appear red.
Entanglement is a Quantum Physics phenomena when two particles’ states can be “entangled”, thus giving birth to phenomena such as Tunnelling (Mossman mentions that in the beginning of HL2).
Dark Energy is a hypothetic energy that has not been proven to exist, but it would explain many things in physics.
Zero-point energy is the lowest level of energy a system can exist at. The Gravity Gun is an obvious reference to that.
These are all the references I can think of, I may be wrong with some, but basically as others have said, HL is a science-fiction themed franchise, and it is often mysterious in nature, which is why it often references subjects that today’s scientific theories cannot explain yet, such as quantum mechanics.