Please note that this is not based on any cited source but on my own observations.
Colloquia and seminars both happen in an academic setting. At my university we have a weekly physics colloquium that — in general — is geared to a well educated, but not specialized audience. (I.e., a particle physicist will present a topic on a fairly advanced level, but so that, say, a condensed matter physicist will still be able to understand). In general they seem to be more "populist" and less technical, covering popular topics in physics (quantum information, graphene) and in the news (global warming, nuclear weapons/power) but from the perspective of a scientist.
A seminar on the other hand, in an academic setting, is a much more specialized meeting, also with a formal academic presentation. For instance, there are weekly seminar meeting for the Atomic, Particle, String Theory, Condensed Matter, and Astrophysics groups. There is also an invited speaker, but the audience is much more technically versed and the topics tend to be much more technical or specific to the field. Generally someone from outside the field will have trouble understanding a seminar presentation.
In a grand sense the two words are equivalent, but a colloquium, as pointed out, is literally a "conversation" and in general has a connotation of being more broad, more accessible, or on a more popular topic.
For the most part, the words are interchangeable. Distinguishing between multiple examples of such things can be aided by their individual connotations:
crack
a line on the surface of something along which it has split without breaking into separate parts
A crack tends to be a visible flaw that can splinter or spider into larger cracks with many smaller, attached cracks. The defining point of a crack is that the cracked object is still together — no matter how tenuous. Often there will not be any visible negative space or hole. Most cracks can be felt by rubbing your finger or hand over it.
slit
a long, narrow cut or opening
The biggest difference between a slit and a crack is that a slit implies some form of opening. Cracks also tend to be on a surface; a slit can be in virtually anything. A stereotypical slit is one that causes some portion of the object to pull apart — large slits can go all the way through to reveal a clean hole.
crevice
a narrow opening or fissure, esp. in a rock or wall
A crevice is typically reserved for very large objects that have been separated into more than one distinct section. A crevice in the earth would be something that separates two plains; a crevice in a wall could very well separate the wall into two pieces. "Narrow" is respective to the larger object. A crevice could be meters wide or only inches. A typical crevice is also very deep.
split
a tear, crack, or fissure in something, esp. down the middle or along the grain
A split more directly conveys an object being split into smaller pieces or a longer top-to-bottom crack. A split could also be used to describe pieces that are no longer attached at all. There is no implication of depth with a split; the importance is the length of the split or how much of the surface remains unaffected.
cleft
a fissure or split, esp. one in rock or the ground
When I think of a cleft I typically envision a V-shaped hole. A "cleft in the rock" is a V cut sideways into a mountain that can provide shelter. The "cleft of the chin" is the middle gap in some people's chins. Depth is important to a cleft but the gap will not go all the way through.
To directly address your example of a flaw in the bottom of a bowl, I would expect crack to be the most applicable. If there is a chunk missing but no thin lines it could be best described by chip:
a small piece of something removed in the course of chopping, cutting, or breaking something, esp. a hard material such as wood or stone
Best Answer
From New Oxford American:
From Etymonline:
Here's some generalizations gleaned from the above:
As to your questions, I'd say they're not quite interchangeable. Knowing their different connotations will likely prove one to be the better word choice.
A dark horse might also be called an enigma, or simply an unknown; while surprise hit, cult classic, and box-office success are other phrases often applied to sleepers.