After 2 1/2 years of bickering with Egypt over practical details of West Bank and Gaza Strip autonomy and, for the most part, sidestepping the basic question of what autonomy really means, Israel now feels that both time and its options for a favorable declaration of fundamental principles on the issue are running out.

In three months, Israel is to relinquish to Egypt the remaining occupied portion of the Sinai Peninsula. Fearful that the impetus for Egypt to make concessions on autonomy, or even to sustain serious negotiations, then will be lessened, the government of Prime Minister Menachem Begin is anxious to reach a broad--even if vague--written understanding with the United States and Egypt that would ensure a continuation of the peace process.

But the basic differences between the two start at a point no less formidable than the definition of the word autonomy. For Egypt, it is a phase leading toward self-determination by the Palestinians in the occupied territories, preparatory to independence. Israel contends that autonomy is merely the limited right to exercise certain administrative powers prior to an Israeli assertion of sovereignty.

Under the Egyptian-Israeli peace treaty signed March 26, 1979, negotiations are to continue until agreement is reached on terms of a five-year transition period for the territories, after which their permanent status is to be decided.

Without a written declaration at least on what autonomy during those five transitional years will mean, some of Begin's advisers feel, Israel would be subjected to increased U.S. pressure after the Sinai withdrawal to make substantive concessions-- pressure that the Reagan administration has seemingly been reluctant to apply for fear of provoking Israel into postponing its final withdrawal.

Also, it is felt here that attention would be refocused on alternative peace plans for dealing with the 1.3 million Palestinians in the West Bank and Gaza, such as the European initiative or the Saudi plan, which Israel calls unacceptable.

Egypt and Israel also are far from agreement on the basic question of the source of authority of a proposed autonomous council that would control governmental functions in the occupied territories, much less the council's size and specific responsibilities, officials here said.

In the dispute over the source of authority of the autonomous council, Israel says that its occupation government in the West Bank and Gaza Strip should be the source, while Egypt says the source should be the Camp David peace accords.

Although the size and responsibilities of the council have loomed as enormously difficult issues since the start of the autonomy talks, it appears unlikely that they would be part of any breakthrough in time to be included in a declaration of principles before the April withdrawal.

Egyptian President Hosni Mubarak's government has made it clear that it will not be pressured into accepting any declaration of principles that would gloss over the differences on autonomy. Foreign Minister Kamal Hassan Ali told Secretary of State Alexander M. Haig Jr. upon Haig's arrival in Cairo that Egypt is not seeking a formula that would "deal evasively" with the Egyptian-Israeli differences.

An Israeli Foreign Ministry source said the Egyptians "interpret the essence of autonomy in a different way than we do, and this will have to be resolved if Haig's visit is to be successful . . . . For Israel it is autonomy itself. For Egypt, it seems to be a phase to something else."

One major obstacle for Haig to overcome appears to be Israel's narrow interpretation of the meaning of autonomy, which, according to a Foreign Ministry position paper, includes the notion that "the central government always retains control over the autonomous authorities by using some or all of the following means, among others:

"It has the power to appoint and dismiss some of the major organs or officials of the autonomous body; it has the right of veto over important decisions; it has certain prerogatives with regard to the budgetary expenditures; and it has an inherent right to apply sanctions when the organs of the autonomy exceed their powers."

The central government, in this case, is Israel.

Israel has not changed its position that the size of the autonomous council should be roughly equivalent to the number of administrative departments in the occupied territories, such as agriculture, justice, education, culture, labor, health, social welfare and commerce. It has proposed a council of about 15 members.

Egypt, on the other hand, wants a parliament-like governing body of about 50 members empowered to enact legislation.

Another major hurdle for Haig is the question of whether 100,000 Palestinians who live in East Jerusalem, which was annexed by Israel after the 1967 war, should have the right to vote in the autonomous council election.

Egypt is insisting on voting rights for them, while Israel contends that this would imply that East Jerusalem is part of the West Bank, not an integral part of Israel. Begin has said he will not bend on the voting issue.