Lieutenant

Lieutenant is a military rank, the equivalent of which is used by the service organizations of many civilizations. As a traditional naval rank, lieutenant is the grade between ensign and the commander grades.

In the Earth Starfleet, the rank of lieutenant was directly between ensign and commander. The Federation Starfleet uses additional grades in this transition, lieutenant junior grade, lieutenant and lieutenant commander. This is identical to the system used by many older Earth agencies. Starfleet lieutenants are expected to be effective small-section leaders and technical experts, developing their leadership skills in preparation for promotion to the commander tier.

From Memory Alpha -- the free Star Trek reference