Word-Choice – What Do You Call Something That Is Not First in a Sequence?

It seems they are called Unpaired Words (maybe the best definition) or Absent Antonyms.

Unpaired Words
An unpaired word is one that, according to the usual rules of the language, would appear to have a related word but does not. Such words usually have a prefix or suffix that would imply that there is an antonym, with the prefix or suffix being absent or opposite. Many unpaired words are the result of one of the words disappearing from popular usage, though others were never part of a pairing and just begin with the same letters as used in common prefixes. The classification of a word as “unpaired” can be problematic, as a word thought to be unattested might reappear in real-world usage

You can find some examples here, but I'll list the majority of them here anyway for easy reference.

1. Words with no positive forms:
   Debunk; defenestrate; dejected; disconsolate; disdain; disgruntled; dishevelled; dismayed; disrupt; feckless; gormless; impetuous; impromptu; inane; incessant; inchoate; incognito; incommunicado; indomitable; ineffable; inept; inert; infernal; inhibited; insidious; insipid; insouciant; intact; invert; misgivings; misnomer; nonchalant; noncommittal; nondescript; nonpareil; nonplussed; unbeknownst; ungainly; unswerving; untold; untoward.

2. Words with uncommon positive forms:
   Disarray; disconcerting; immaculate; impeccable; inadvertent; incapacitated; incorrigible; inevitable; innocent; inscrutable; insensate; insufferable; interminable; unbridled; unflappable; unfurl; unkempt; unmitigated; unrequited; unruly; unthinkable; unwieldy.

3. Suffixes (asterisk means "word not existing"):

   • Reckless/*Reckful
   • Indefatigable/*defatigable -> fatigable
   • Flammable-Inflammable (not antonyms)

There are a lot of terms being used in your question. In an attempt to make things a little more clear:

antonym — a word opposite in meaning to another (e.g., bad and good).

The first thing to remember is that "opposite" here does not strictly mean "polar opposite on one particular scale." With regards to late, on-time and early are both antonyms.

The second thing to remember is that antonyms are not logical constructs — they are still linguistic constructs. The implication that two words are antonyms of each other is not inherently wrong but it may muddle the truth a bit. For instance: early and late are antonyms of on-time but it is entirely possible that the inverse is not as true. Is on-time really an antonym of early? Perhaps, but it causes me some pause.

sequence — a particular order in which related events, movements, or things follow each other

In this case, the sequence is variance between arrival and scheduled arrival. The center of this sequence can be called many things:

• midpoint — the exact middle point
• middle
• halfway point
• midway point
• center
• balance

And so on. Most of these words are used for a more strict definition of sequence or to explain a point between two distances.

In terms of looking at things as a sequence, the term antonym drastically shifts meaning since these things are much more logical or mathematical. Specifically, what is opposite depends a great deal on the type of sequence being used.

Unfortunately, the more picky one gets about opposites, the less the term antonym applies. If two towns are across the river from each other they are opposite each other but they are most certainly not antonyms of each other.

neutral — having no strongly marked or positive characteristics or features

My dictionary is not doing this word justice, unfortunately. Neutral, in your usage, implies the halfway point you described from earlier. But in your example, on-time fulfills more of a pinnacle or balancing point:

• fulcrum — a thing that plays a central or essential role in an activity, event, or situation
• accurate

The act of being late or early is missing the goal of being on-time. Represented as such, late and early are antonyms of on-time and could be considered opposites of each other in the same sense as the two towns across each other from the river.

Imagine a mathematical tree drawn out with the tip being on-time and the two children as late and early. A line connects late and early but only by traveling through on-time.

All of this is to lead into the actual answer to your question:

What is the relationship of on time and early?

There are two ways to answer this question: One is to look at the relationship between on-time and early as they exist in a sequence and the second is to look at them as potential antonyms.

The difference between them with regards to the sequence is strictly one of numbers: on-time would be the goal and early is someone not hitting that on the nose.

In regards to them as potential antonyms, one could make the case of them being antonyms of each other.

The question you may have been trying to ask was:

If late and early are antonyms, then what is on-time?

The answer is that on-time is also an antonym.