Slightly wider somewhere in the middle than at either end; often the wideness is not quite centered.
An object with this sort of shape will get described differently depending on how much longer than it is wide: something now that is not much longer than it is wide will usually be called clavate, while something that is longer one will be called tapered. In fact, in some cases, clavate shapes (like the hymenium cells in the picture here) are almost ovoid.
Tapered can also refer to macroscopic structures that are simply wider at one end than the other. Except for this case, clavate and tapered objects can also be called ventricose, although that term also refers to shapes where the swelling is quite sudden, and perhaps irregular. Clavate and tapered are only used in situations where the increase in width is smooth and gradual.
This term is used in two senses:
1) Long, coming to a (usually blunt) point at both ends and swelling gradually from them all the way to the middle.
Of the four septatespores lined up at the bottom of the picture, the right-hand two are fusiform in this sense (the extreme right one being a pinwheel of fusiformspores)
Many people defining this term give "spindle-shaped" as a synonym. Why they would think this useful, when spindles haven't been in common usage for more than a hundred years now, totally escapes me. For the record, a spindle is shaped like usage number one here.
One can see from the picture why one might want to have both usages available: it means you only have to use one term in describing both spore shapes of this species. However, I believe that the pointy hotdog shape should be designated with the term acerose, which specifically means that shape and only that shape; fusiform can then be reserved for objects that have the "genuine" spindle shape.
The usage of fusiform (in the first sense) also seems to overlap with the usage of ventricose (in its first sense). The issue there seems to be how gradual the central swelling is. In both cases, the thicker "center" may actually be quite off-center.
And then of course there's amygdaliform...
"Lance-shaped". I have the quotation marks because (in spite of the real-life shape of a lance) when this term is used in mycology it tends to denote cells that have a wide, rounded base and come gradually to a point, like the brown setae in this picture of Phellinus rimosus.
For a cell that really is lance-shaped, use subuliform or subulate.
Swollen in the middle. This word is used in two senses.
1) The first sense is what you would expect just from reading the definition, as shown in the picture of the protruding, ventricosebasidia of Lactarius piperatus.
2) The term is also used when something is only swollen on one side, giving the ventricose object a profile like a person with a beer-belly. This usage is most often applied to spores. For all other objects, assume the first usage.