(written from a Production point of view)
Harvey P. Lynn, Jr. (3 August 1921 – 1 December 1986; age 65) was a RAND Corporation physicist and the special scientific adviser for Star Trek's pilot episode, "The Cage". Lynn was brought to the attention of Gene Roddenberry in late May 1964 by US Air Force officer Donald I. Prickett, a World War II comrade, whom Roddenberry had written for technical advice, concerning the scientific and technical accuracy to which he wanted his new Star Trek production adhere. (The Making of Star Trek, pp. 75-76) Using his own personal time, Lynn served almost a year and a half as a science consultant for the series, receiving a $50 compensation per episode.  For the regular series, Lynn was succeeded by Kellam de Forest.
Lynn took exception to some of the jargon used by Roddenberry, such as "laser" and "rocket", because these were devices already in existence in the 20th century. Originally, the episode was to take place in the Sirius star group, but Lynn suggested that another star system be used instead. Lynn is often credited with inventing the term "phaser". Most of Lynn's scientific objections were outlined in a long memo he sent to Roddenberry on 14 September 1964. The full text was reprinted in the 1968 reference book, The Making of Star Trek (pp. 90-95). Lynn was one of the people who scrutinized Matt Jefferies' work, while the latter was in the process of designing the exterior look of the new starship USS Enterprise. (The Making of Star Trek, p. 80)
His son, Harvey P. Lynn III, has commented this in 2002 about his fathers involvement, "He graduated as an Electrical Engineer. Worked at RAND as a liaison Officer between RAND and Project Airforce. Was never starstruck and had little interest in TV, films, or science fiction. Apparently he met Mr. Roddenberry though a mutual friend and was selected for the technical consultant job more because he hit it off with Mr. Roddenberry than his technical expertise. When offered the job, he boned up on physics, astronomy, etc. He picked up surprisingly quickly on how to express the technical elements simply... ie not having to explain how a phaser works... sort of how most people know that a light switch turns on the lights but don't wonder about the mechanics. From his earnings as consultant, we bought our first color TV. I think that's in The Making of Star Trek. My Dad was never really a big fan of the show... not a Trekkie in today's sense. I've retained only about 3 scripts. PS. My family was really moved by Mr. Roddenberry sending flowers to Dad's funeral in Texas as they hadn't really been in contact for years." 
Lynn died in December 1986 (on New Year's Eve, according to Robert Justman, but he, presumably having gleaned the information from an obituary, was off by nearly a month, ) only months after his eagerly anticipated retirement. (Inside Star Trek: The Real Story, p 442) According to a former co-worker, Lynn had planned to relocate to his native Texas,  and where, as stated by his son, he was interred. Decades later, Andre Bormanis followed in the footsteps of Lynn in pretty much the same capacity for the modern, spin-off Star Trek live-action productions, while Jesco von Puttkamer served as such on the earlier Star Trek: The Motion Picture.
Further reading Edit
- The Making of Star Trek by Stephen E. Whitfield and Gene Roddenberry
- Inside Star Trek: The Real Story by Robert H. Justman and Herbert F. Solow
- I'm Working on That by William Shatner with Chip Walter