Came across a great definition while reading up on release 4.0 of WordPress:
A release lead, if anyone is curious, determines all important parameters for a release, like schedule, deadlines, which feature plugins are merged, and more generally, scope and goals. They take point when it comes to meetings, shepherding contributions, announcement posts, and updates. A release lead is a connector and facilitator, identifying bottlenecks and friction wherever they may be and at the service of the developers and plugin teams that are aiming to have something in a given release, and be in frequent communication with them.
The release lead should should follow what’s being committed, and set the tone for prioritizing and gardening the milestone on Trac. Given the constraint of time in hitting a date, help with prioritization and ensuring good communication lines are two of the most valuable things a lead can contribute.
The last five release leads were lead developers, but that’s not a requirement, nor is being a committer. I always thought of my “code reviewer” and “committer” hats as being separate, additional responsibilities. (Helen, of course, also wears these same hats.) Regardless: the release lead has the final call on all important decisions related to the release.
I particularly like the term "gardening the milestone" as this is a very suitable analogy to what the responsibilities of a release lead involves as it's very much an art like pruning a rose bush or a hedge and keeping a garden looking sharp.